Sims register through the Administration Center RH Rug, not with the phone or computer. If you don't have the rugs yet, you can get the University RH replacement rugs at the creator's website. Look for the link here: Simsl3gacies Downloads
The first campus I used was one I found at MTS. The lot is 50x50. It was built by lewisb40. I made a few changes, but not many. University for your Home World
I added the small student union building in the parking area just to see if I could get the Meet and Greet objects to show up (for fun), but that building isn't necessary for sims to attend classes or graduate. I also added a small indoor garden area on the top floor of the Science building. The original lot has no CC. But you'll need the rabbit hole rugs and NRaas Career mods. The University expansion is necessary, of course. Some items from the stuff pack 70's, 80's, & 90's were used too, but the creator said those aren't necessary for the lot to function.
The campus itself does not have to be elaborate. I also converted a 3-story home into a "community college" in another world. The RH rugs and Careers mod are the main things required. There are 5 rugs, and you just need to make sure you have about 5x5 clear squares of space for each rug. Designate the lot itself as a Hangout or as Visitors Allowed.