Re: Sims University

"Enrolling in university can be done at any time via the phone or the computer. Players will be able to choose their Sims' major and how many credits they can apply to, which affects the length of time the Sims will spend at university. The Sims may only study one major at a time, but they can enroll in university after completing one major in order to study another. They can also customize the duration of the Sims' term in university,1 week is the min and 2 weeks is the max."

http://sims.wikia.com/wiki/The_Sims_3:_University_Life

Hopefully that helps, I'm assuming you're confused about how you enroll your Sims into uni. <3