Sims keep leaving party around the same time every night

This may be intentional, especially if the guests' needs are not being met. Are you seeing any telltale thought bubbles above their heads before they try to leave that may indicate boredom, hunger, low energy, etc.? The inactive sims at Uni World are notorious for not being able to take care of their own needs consistently, so they may have arrived to the party that way or they may need some coaxing to join in any activities you may have planned for them if they aren't going to find their way to the food, music for dancing, the pool, or each other to socialize (assuming they don't actually hate the other guests) by themselves. Is there entertainment going on? Can you chat them up a bit as they arrive and bring them each, or as many as you can reach, a drink or a plate of food?

If you are using mods and have NRaas MasterController, you can get a reading on their motives (needs), if that helps figure it out.

I'm not sure what the problem was. I tried everything I could think of. There were no locked doors, the pool was accessible and almost right when they arrived I had them eat first. I also had plenty of stereos throughout the house playing music and 4 kegs. The only Sims that stayed were the 3 that were playing a game of pool with my Sim. Oh well :/ Those Sims stayed for a while at least. Thanks Igazor.

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?

If you are inviting sims that are scheduled to work at university (cash register attendants, mascot etc) they will show up at the party but leave soon after.