Agreeing to extra hours

I have worked at a nursing home for 6 weeks now. When I was hired, I told my employer I would like to work three 8-hour shifts per week. Since I was hired as part-time, I was told this would not be a problem. However, I have been scheduled to work 4 days per week more than I have been scheduled for 3. Although this has caused me some stress due to getting behind on school work, I have not said anything. Everything else at this facility has been great. The co-workers are welcoming and helpful, the nurses do not belittle the cnas (with the few exceptions), and the patient to cna ratio is not bad. School will be over next week and I was still able to achieve a 3.3 GPA, so it didn't turn out too bad. However, in the 6 weeks that I have been there, I have worked four extra shifts that I was not scheduled for. Twice I was asked (and agreed) to work for other CNAs and twice I was called a few hours before the shift started due to someone calling in sick. Last night another co-worker asked if I would be willing to pick up a shift for her on my weekend off (each employee is scheduled to work every other weekend). I have yet to give her an answer. Since I am new at this job, I am not sure how to handle this. I don't want to be uncooperative, but at the same time I don't want to be the person that everyone runs to when they don't feel like working their shifts. I have worked three weekends in a row. I work second shift, and would enjoy a weekend off to catch up on time lost with friends and family. Any advice?