How to send a sim to university, then play as the original household instead?

imlividd​  With Traveler, you can take a sim on vacation to any world, University included, through the phone option.  You don't need to enroll them in University, and doing so might make the process more complicated anyway.  You can also enroll them once they've arrived in the University world.

Anyway, travel with the sim to that world, enroll them if you like (or don't), pick where they're going to live, let everything load and run for a couple sim-hours, and then tell them to return home via phone.  Except once you have the travel screen up, double-click their name to remove them from the list of sims traveling.  You'll wind up in what's called an empty travel scenario, and once the homeworld is loaded again, you should be able to select your main household to play with.

To bring the other sim back, you can empty-travel to their world, select them, and have them travel home.  But it may be easier to use MasterController's Add Sim command on a lot (your main one or a different one) in the homeworld.  If you don't see them on the list of options, filter by homeworld and choose the University world.

I would strongly suggest doing a test run before you play through this scenario.  As in, send the sim away, return home, use "save as" so you don't overwrite your main save, quit, reload, and see whether the Add Sim command works.  If it doesn't, then you'll need a different approach, and it's much better to find out now than after you've played for a sim-week or two.