!!! Please do not upload my mods to other sites. It is also stated in the permissions.
This is the only place where I can support.


I write this at the top before the mod description as I think it is important:
EA has good reasons for having the 20 sims on lot limit.
If you go beyond that your computer has of course more to compute, so increase the number of classmates reasonable.
(For my computer 40 sims on lot is ok, but 50 is starting to lagging and the cooling fan is running all the time). 
Maximum sims in Zone for high school lots is set to 100. So in theory you can have about maybe a bit more than 80 students.
(Rough formula for calculation: Number of students + 4 teachers + 4 cafeteria + 1 principal + 1 janitor + 2 firefighters + number of teens in your played household + maybe the Reaper => number of students + 20 should be sufficient)

If this is TLDR, there is a really good tutorial video from KimbaSprite linked at the videos tab.

Increasing number of Class Mates:
You have to choose your classmates, else none will show up at school. (You can go alone to school if you want.)

Click on the mail box of your lot, select "High School Students ..." ( see the first screenshot of the mailbox)
When you do this the first time you will only see "add ..." and "Faculty ..." in the menu.
Add students via "add ..."and the Sim Picker that opens. (Up to 40 at a time, but if you want more simply reopen it a second time)
Then again select "High School Students ...". You should see now the "Classes ..." menu. In there "Class 1 ...", "Class 3 ...", "Class 4 ...".
Add students via "add ..." to them.
Via this you can split your class mates into four classes. Class 1, Class 3 and Class 4. The others will go to Class 2.
(You can build a mailbox on the school lot if you want to reassign your classmates in the breaks for the next lesson.)

Up to 4 classes depending on Whiteboards:
If you add a new teacher to the faculty he will be assigned to all classes.
You can remove them from classes via the faculty -> teachers -> class X -> remove
Multiple teachers per class and multiple classes for a teacher is possible
So you can have for example 3 teachers for class 1, 2 for class 2, and maybe only 1 for class 3 and none for class 4.
If you do not assign a teacher to any class he will be used only for the events like prom etc.

Before you go to school place 2 or more additional Whiteboards on the lot.
After that assign them to class 3 and 4, by clicking on them.
Hint: If you want to reassign the whiteboards, place a fifth one for a triangle exchange,
You can reassign boards if you want a class to go to another room for the next lesson.
For this you need more boards than classes on the lot.
The determination of the white boards is done on zone spin up. Therefore best, if you add whiteboards do this before lessons, then leave the lot
and come back for school start. Also the number of needed teachers to stay on the lot is decided at the start of the school event.
(If you do the edits during the event it is too late. They will be active the next day)

Pranked boards seem not be assignable anymore, simply replace them or do a shift click => reset object on them. (EA issue)
Pranked boards need to be cleaned by the teacher before the students can go to class room. (EA issue)
(To disable pranking by NPCs you can use MCCC -> MC Tuner -> enable autonomy scan. And than click on a whiteboard and disable the
corresponding interaction)

Faculty:
For changing faculty members there is the "Faculty ..." menu. (See above for screenshots of the mailbox).
You can select for each faculty member multiple sims. They are randomized in the morning btw. start of the events.

Teachers are also used for several events. Best select at least 4 for the 4 classes plus 2 for the football and cheerleading event.
(For the initial teacher 2 from the High School package, best remove him once and add him back)
New teachers are not created. So you should assign them via the mailbox menu. Else there will be none.
After adding the teachers to the faculty also ensure that they are assigned to the classes they should teach.
and remove them from the classes they should not.
For example, if you play only with two classes, remove them from class 3 and 4. (also for performance, as sims do not need to search them then)
If a class has no teacher assigned of course none will show up for the class.

Principal: There should be only one, but atm you can have multiple. But I think only the first one will apear.

Cafeteria: You can have now 4 cafeteria employees on the lot. (You should place cafeterias in build mode else the vendors without a work
station will leave)
New cafeteria workers are not created. So you should assign them via the mailbox menu. Else there will be none.
The first cafeteria will open from (7)8:00 to 18:00, additional cafeteria workers will appear at 10:30 and will leave at 14:00.
(At firedrill days they come a bit earlier)


Prom, Football and cheerleading events:
Only your choosen class mates will appear, but all of them.

Career day: Atm restricted to 3 groups of max 10 students. (Carrer days are boring.)

Core Students:
There are 5 so called Core Students. 4 of them should belong to the 4 different school activity clubs. (Chess, E-Sports, Football, Cheerleading. For the football and Cheerleading events you will need even more. (About 4 or 5?)
One is the bully requiring the "Mean" trait. (He is not really needed I think. I play without one as the one I had made everybody tense)
For normal schooldays they are not really needed, but nice to have them advertising their clubs after school.

Assigning NPC Students to the different clubs: You can add and remove the students to the 4 different school activity clubs (volunteer careers as defined by EA) via the mail box menu for students. If a student is not selectable, he may already have a volunteer career like pathfinder or theater whatever. (Remove their volunteer career via mccc or by playing the houshold and quiting it.)

Install:
Simply unzip the downloaded file and then copy the directory to C:\Users\YourUserName\Documents\Electronic Arts\Die Sims 4\Mods
Activate in game Script Mods allowed in the Options.
If updating from version 4.6 or lower. Replace the boards in schools in build mode.

Optional Files:
        Override Files:
I recommend to read this: Simplified Modding (tumblr.com)
Place the resource.cfg (I attached one in the optional files) in your Documents\Electronic Arts\Die Sims 4\Mods
Create an "Overrides" folder in Documents\Electronic Arts\Die Sims 4\Mods
Drop the optional file into the Override Folder.

Compatibility to Chipped School Tweaks: z_MCM_ST_compatibility.package : only needed when using Active Classes part of School Tweaks.
Get ready for Class earlier: z_MCM_goEarlier.packageGet ready for class starts at 9:15 and 13:15
Get ready for Class later: z_MCM_goLater.package     Get ready for class starts at 9:45 and 13:45
NoCLubOutfitsDuringSchool: z_MCM_noClubOutfits.package.

Normal files: (Can be placed anywhere in the mods folder.)
No_Autonomous_CheckClassSchedule: Prevents teens to use locked computers. Just a workaround by disabling autonomy, not a bug fix.
ExpellFix : Fix for [NEEDS SAVES] [HSY] Randomly Expelled/Teen gets expelled from daily - Answer HQ (ea.com).
This will enable you to reenroll to high school without being expelled the next day without reason again.
It will not prevent your teen of being expelled.
Fixed in Patch 1.98.127/1.76. I'll remove this if it is confirmed to work really.
NoNPCAutonomy: Contains 5 files to disable the annoying NPC relationship autonomy to ask the played sim to be Best Friend, Girl-/Boyfriend, to prom, to prom as friends and propose.

Hints/FAQ:
- If the menu does not apear on the mail box and the "Aramiteus_HighSchool.ts4script" is not apearing in the mod scripts list, shorten the path name wether by moving the "Aramiteus_HighSchool.ts4script" down to the
mods folder or by renaming it to "Core.ts4script". Or whatever you can do to shorten the pathname of the mod files. 
Sims 4 has a problem with long file path names.

- Do not use fast forward at school when your event is close to a state change. (Begining and end of breaks or lessons).
The Sims 4 else may miss to trigger the transitions needed for situation change and your sims will do nothing till they get the
trigger for the next situation. (I am not sure if this is even working without this mod in the school with 12 students)

- If the mod does not work at all, you see teens not choosen or newly created at school, check your mods for conflicts.
The less you have the better, as mods are always come at the cost of performance.
In general as usual I do not recommend to use mods at all, besides obvious small bugfixes.
Modders do not have the time and the team size as professionals to test and consider all aspects of a software. 

- If not all choosen classmates, teachers, caterers appear:
Check if they are still assigned as Class Mates, are still teens, do still have the High School Career.
Check also Keep Sims in Home Region by Kuttoe (itch.io) for update if installed and enable the "High School Toggle" of it.
Seems like the "High School Toggle" of Keep Sims in Home Region by Kuttoe (itch.io) is not working for the teachers and caterers.
Move them to regions that are allowed to travel to Copperdale or allow their home regions for Copperdale..
If you have MCCC installed disable teenage can quit Highschool:  MC Careers -> Teenagers quit school.
Set it to disable. (Had the case that teens, did not have the High School Career anymore)
The students are called additionally to the initial call between (7)8 am and 9am. Nevertheless it may happen that not all are called in
(For me, most times the last ones in the list if you push "remove" on the mail box menu.

- if all teens apear at school, but do not move:
Check for performance heavy mods. For example the latest patreon version of Basemental Drugs. Reducing the number of students may also help, of course.

- if the students do not go to their class:
Check if the whiteboard has been pranked. (If you see a smiley on them.). You can reset them by shift click on the board => reset object.
 The teacher normally should clean the board, but it often fails. Pranked boards can not be found by the students.
Assign at least 4 or even better 6 teachers to the faculty.

- for resolving the crowd in front of the school please use Improved Autonomy During Loading Screens | weerbesu on Patreon. (I think
it even improves performance, as it resolves the dead locks sims cause when several sims choose the same interaction target)

- if sims do not appear in the sim pickers:
For me this happens when I add a sim from the library. Seems to be a vanilla issue I can not do anything against. It helped
to delete the localthumbs.cache and give The Sims 4 time (or several reloads) to get known of the new added sim. 
Try to meet them with your played sim the the teens placed down from the gallery (introduce themselves to them). 

- There is no clean interaction clean, on the whiteboards:
Whiping boards is avalable after 18:00. (Vanilla issue see [OPEN] [HSY] Clean Whiteboard not available in interactions - Answer HQ.)
For assigning boards they have to be unassigned.


Incompatible mods:
Zerbu High School Spawn Fix: Mod conflict. Using the same records. Fix is included btw. not relevant if using this mod. 
Rex: Choose your classmates: Feature is included here
Ksuihuh More Students at highschool: Mod conflict. Using same records. Correspond to the old version 1.2.
A good alternative if you want random students.
Basemental drugs, patreon version: Seems like together they cause massive lags, causing the scheduler of the Sims 4 not working any more
This is not a real incompatibility. If you have a high performance computer it may work together. Not much I can do about without as I do
not use the patreon version and meanwhile I rolled back to basemental alcohol improving the performance in general.
adeepindigo's 'delinquent teens': To be confirmed. I have to check what is the problem with it.
Walkby Overhaul from Zero: To be confirmed. I have to check what is the problem with it. Seems to cause no students to show up.
Assign NPC roles by Zero: To be confirmed: All teens from the whole save show up in the school
KawaiiStaci's preschool mod, entire education system bundle mod. Causes endless saving.

My To Dos (not yours, as mentioned in a video):
Next Release:
- enabling selection of vampires for faculty
- optional file for removal of career outfits for teachers and the principal
- enable to appear random janitor from the selected janitor pool
- fix for principal not using his desk during the morning
- optimisations.

after next Release:
- handle the case when sports events starts at 17:00 that students stay at school till 18:00.
(Possible solution: Send students home at 17:00 so the event can call them in again as participants)

Later (Nice to have):
- Assigning teachers to certain lessons. A teacher for Math, an other for Arts, etc ...
- Graduation? I must have one first :-)
- Detention for NPCs. Most time the teacher is supervising detention for no one. I want a "breakfeast club"!!!!
- play as teacher in education career and become principal. (But education career is part of the University pack. Maybe a new
career definition is needed). I am really eager for this feature. But this will take a long time.
- assigning desks or stools would be nice. Or do students change their places each day in real life?

Canceled:
- fun decay during lessons?
- Exclude teens living in the werewolf world, strangerville and cottage world from the sim picker menus.
Seems like they are unwanted for gameplay reasons
- if a teacher has no class to teach as no one is assigned or came to school, he goes home. => But it is possible to assign them during morning activities, maybe some people want it this way)



https://www.patreon.com/Aramiteus