McDonalds hours

I had an interview at McDonalds recently and I didn't tell them that I'd be going back to uni in September, I think they assumed I was going to the uni at my home town and would be able to work throughout the year. I've got the job now and I have a Welcome Meeting soon, just wondering - if I sign the contract am I entitled to work throughout the year? Should I tell them I need to go back to uni? I really need this job so I don't want them taking away the job offer but at the same time I obviously need to go back to uni and don't want to sign anything that says I have to work there full time. Someone please help!