There are a few ways to make it happen, depending on if you want to use mods or not. The no-mods way is to move your families to Copperdale - I do believe kids from there are picked for school first and I believe that accounts for played sims too.
The other way is to add the mod "choose your high school classmates" (available on CurseForge), then you can by clicking any mailbox add and remove classmates. Any teen can be chosen, but you need to make sure the core roles are filled (meanie + one student for each after school activity). In order for a student to be chosen for this, you manually have to make sure they have the specific after school activity first.