HELP! Need to quit my job?!

I work part-time whilst at uni and I absolutely adore my colleagues.

The problem is that my bosses, as lovely as they are, don't respect the fact that I have uni work which takes priority - for example, I was once told I should miss two lectures to work because I could catch up, I once had to run from a lecture to work a six hour shift and missed lunch, they last time I was in I was meant to finish at 3pm but got told to stay until 7.30pm. I also often have breakdowns as a result of the job - I've cried in Tesco because I don't know which food to buy because I'm so mentally drained, I've ranted to many of my friends to the point of tears, and often I return with physical injuries because one aspect of the job can result in an insane amount of bruising on the arms.

I'd like to quit. My friends would like me to quit. But how do I go about this without making my colleagues dislike me or making it too awkward to pop in and say hello again?

Explain your situation and the effects it has on you, and that as they simply aren't being flexible enough to both study and work, that you will have to leave your job.

How many hours do they make you do, and what area of work is it that you have been in?

Hours vary from 6-20 hours a week, sometimes they call the night before to say I've been put on the rota for a next day shift because someone else can't make it.

It's catering - specifically waitressing.