So far, I have a group of teens (12 total including the played teen(s) I send) attending high school. It is the same group all week, Mon-Fri.
Then on Monday, it seems to re-roll which teens are selected, and sends a different group of teens. Then those teens attend the school Mon-Fri that week, etc.
The game does pull from my played households as well as NPCs, but still randomly. For example, one of my other played households has 3 teens, and the game sent only 2 of them, but it was the same 2 all week.
I'm guessing this is by design. If the game sent all teens in the game, the school lot would be unplayably full and laggy. If the game re-rolled who attends school every day, it would be hard to build up relationships at school. This way there's some consistency, but re-rolling each Monday allows there to be some randomness and variety (and a chance for a certain sim to show up if we're hoping for it).
I have not tried changing which household I'm playing during the week. It's possible that attending school while playing one household on Monday, and another household on Tuesday, would cause the game to re-roll the other teens again on Tuesday.
For those of you who want more control over which teens attend, I've heard really good things about the mod mentioned above, people seem to find it really useful. I haven't tried it myself.
