Open PedPop.dat that is located in: \Rockstar Games\Bully Scholarship Edition\Config\dat\PedPop.dat
This file sets how many peds from each faction can spawn in a specific interior. Although it's easy to understand and edit, it doesn't have every interior in the game.
These are the 2 interiors you want to edit:
School_Hallways
12, 0, 2, 0, 0, 0, 0, 7, 0, 0, 0, 0, 3,
3, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
Boys_Dorm
5, 0, 2, 0, 0, 0, 0, 2, 0, 0, 0, 0, 1,
1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0,
4, 0, 1, 0, 0, 0, 0, 2, 0, 0, 0, 0, 1,
1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0,
The 4 rows mean day, day while in class, night and night while in curfew. The first number is the total population that can be in that interior, each time of the day, and the rest of the numbers are each faction.
1 Total population
2 Prefects
3 Nerds
4 Jocks
5 Dropouts
6 Greasers
7 Preppies
8 Students
9 Cops
10 Teachers
11 Townpeople
12 Shopkeep
13 Bully
Make sure the total population is equal to the population of each faction combined.