When in college, I always headed to the place with a cheap small pizze with one topping. Fraternaties and clubs and organizations will get the larger deals, very likely. Students, I would suggest, will look for smaller pizza deals with a discount.
So, you can do both the groups and individuals or groups. Calling the student activities director directly and introducing yourself and your offerings could do you some good. Find out who the key people in administration are who provide programming for students and ALUMNI.