Can I loose my job for wanting to work less hours?

I work for Coral Ltd and I am on a 25 hour contract. I only ever work night shifts and want to cut down my hours as in September I am going to college full time and I think working 5 nights a week plus college might be a bit too much for me and I will need more time for studying as it get closer to exams etc. I would like to switch to a 17 hour contract but I am worried that they will refuse to let me do this because when I started the job I wasn't in college and they say "work comes before anything else". Can they refuse to let me work less hours? Can I loose my job because my avaiability to work has changed? Bearing in mind that now I only ever work night shifts so I still would be able to work them I just don't want as many. Help!!