This mod fixes an issue that prevented player-made Sims from spawning at the high school in High School Years. Additionally, it makes it so that the game is equally likely to choose Sims that are assigned to either class 1 or class 2. The two issues are related, so it's not easy to fix just one.
Please note: This only fixes the issue preventing player-made teens from spawning, it doesn't guarantee that they always will. The game uses its own system for deciding which Sims to spawn, and it may or may not choose player made ones.