I went through that kind of thing in college...mostly because I wanted parttime hours and the managers would continually give me fulltime or call me in and pressure me to work more often. <br />
I think that you might start looking around for other work. And then have a sit down with the manager. Tell her that you feel you have done a good job, but that your studies are very important and must come first. Then very calmly explain that you can only work during certain hours...with no allowable variations. Tell the manager that you can't maintain the schedule that you have been given in the past, and ask if she is willing to work with you. If she says "yes", then hold her to it. If you are put on the schedule during a time outside of the hours you have specified, then simply tell them that you can't work that day. If I were you, I would present the manager with a written list of which hours you are available to be slotted. <br />
Some businesses find it very hard to work around everyone's schedule, and it might not be the manager's fault. But you have to decide what your priorities are, and if they can't allow you to uphold those, then you might have to find another job. Some places will be more willing to work with you. Others won't. It really depends a lot on how well you work and how easy you are to replace.
Do the best you can with what you have until you find something else.