Manager wants me to work until closing shifts even though I told her my ideal times.[Serious answers]

I started my new part time job today. It’s not bad, and everyone is nice, including my manager. It’s pretty stressful since there’s a not many people, but they all work hard. The only thing is that I have repeatedly told my manager that I can work only three/four hour shifts on Fridays and Saturdays, yet she is trying to make me take closing shifts on both days. I have school to focus on, but I needed a part time job so I took this job. I seriously don’t know what to do. I don’t want to seem like a bad employee because I literally started today. She’s nice, but what can I do? Should I just do closing shifts on Saturday only? Please serious responses only.