Technical stuff: this is actually pretty simple. So, for these events (which are called Situations), the game grabs two different types of Sims. The first type is Sims who are actually enrolled in the Spirit Squad, which I didn’t touch. The second are Sims that aren’t even enrolled most of the time. You can tell who these are by the “Student” or “Party Goer” tooltips above their heads—they aren’t actually students, they’re random Sims.
Both of these types are assigned a “Situation Job” tuning file by the Situation tuning file. Therein, the game is coded to select Sims based on a filter, which is called a Sim Filter. The Sim Filter that was there for Sims in the “Student” Situation Job was just a general one for anyone who was a Young Adult or Adult. This mod changes that filter to one that serves the same function, but only for Sims who have never been played.
The same was done for the “Party Goer” Situation Job, used in Game Day situations (which is on Fridays).