I heard (not sure where) that someone came out with a mod that allows a player to put/build a high school in any of the towns in the game.
But I do know a mod that allows you to have more than just the eleven students at school.
https://ksuihuh.wixsite.com/corner/post/_more
The choices are 18 students, 24 students, and 40 students. I've tested them all, and I like the 18 the best. The 24 students mess up the school day a bit, and 40 students messed up the whole day. In that is also another mod that allows an extra food vender, which really helps with more students regardless of the number you go with.