The more you expand your campus and classrooms, the more students your university will be able to have. You will earn money with each hourly tuition, which will be higher every time you upgrade your university. 

 

You also need to take into account your expenses. If they are too high, maybe you should consider reducing some staff.

 

Another way to get some extra money is in the University competitions at the end of every school year (yellow cup icon at the bottom right corner of the screen).