I find that many of my schools blurp everyone outside every so often and then, if I don't notice soon enough, the students may wander away and start doing other things. It seems to be worse (though this could be my imagination) if I happen to save, quit, and later reload during the school day. This also happens to a couple of my other rabbithole lots where many sims work.
If you have MasterController in play, NRaas > MC > (Sim) > Intermediate > Career > Go to School. There is an equivalent command for Go to Work on the same menu. (MC Cheats add-on module required)