A "solution" that requires no mod, but other expansion packs involve the usage of the club system from Get Together. I say solution lightly as the sims will not attend class as normal, however this has been the only way I have found to bring other Sims to the highschool lot. To do this, create a club with the teen sims who you want to appear at the school. While attending school with your sims select a sim who is apart of the club, and not at the lot already, and choose to invite the club to gather at the lot. This will bring all the sims from the club to the lot, but as I said earlier they will not attend class as normal. This has been the best "solution" I have found as I don't enjoy hopping though the hoops of ensuring that cc works and is up to date.
I play rotational with about 15 different households in one save. There are two or more teens of my played households attending school, but they're always in class 2 and never in the same class as the active teen.
