Depending on the talents and Traits of your Sim, taking the Aptitude test before enrolling into university can get them extra credits or small scholarships. If you're playing normally I definitely suggest taking the test in order to give you as much of an advantage.

When you first move in a university mascot will come by and drop a green bucket with university goodies in it. This is where the interaction to take the test it. If you sold or deleted this you can purchase a new one from Buy Mode. The fastest way to find it is to go to 'Collections' and then to the University collection.

Note: it will help to level up your skills before taking the test.