This is just a wild guess, but since it's Uni World don't you know that Uni students like to have a bubble bath, a cup of hot chocolate, and go to bed by around 9pm so they can be fully rested and have time for a proper breakfast before classes for the next day?
No wait, that's not right. What I meant to say was that sometimes the Science Experiment in the middle of the campus Quad can be a huge draw for sims, supposedly depending on their interests and majors but I think I've seen sims of all types get pulled into tinkering around with it. There could also be something else going on that is pulling many of them away. Have you happened to notice if everyone just goes home (or back to their dorms) or elsewhere after they leave early?
