Being a student myself at a fairly large university I can personally say that if it is at all possible, limit your work hours to a minimum. I go to school with many other students who are all attempting fairly high course loads (18-21 credits), many of them trying to go the pre-professional/pre-med route. Simply put, if you're trying to go for a 3.7 GPA I would say working is a dream. Most of the students I go to school with don't work at all, or very little, and only at jobs that allow them to make their own schedule. Depending on the type of school you're going to this will probably be assumed. Most professors could care less if you need to work to put food on the table, and will in turn assign work loads to accommodate the mass amounts of free time they expect of a professional student. I would say, depending on the rigor of the institution, up to 25-30 hours a week of studying may be expected per class. Not all classes are like this and it depends entirely on your aptitudes
Depending on where you go to school, I think you'll also be surprised by the number of students who simply assume large amounts of debt. I know out-of-state students who, in order to succeed, will eventually be in $80,000 of debt and they accept this because...well it's kind of expected. Outside of this you either have kids on a full-ride or have their entire education paid for by their parents (more common than one might think). This is just for undergraduate.
University, from what I've seen, has become more and more of a type of "experience" and right of passage that is expected by employers. The life skills learned in an institution of higher learning are really indispensable and the money is absolutely worth it. If you must work, I suggest finding a job that will let you work perdiem as some have suggested. Some people I know work for local private transports and make decent money working 20-25 hours a week. They don't call them poor college kids for nothing. A lot of the experience is how to live and entertain yourself on next to no money.