Is this a reasonable request of I'm being asked to work extra hours last minute?

I am back in college again, and currently have four different babysitting jobs on the side. Two are consistent (same hours every Monday and Friday in the school year). The other is only in the summer or if the parents go on vacation. The other job though, has really inconsistent hours. She's got five people working for her, and a special needs child who is completely immobil and non verbal. I'm not really one to sit and do homework the whole shift. Sometimes a littlerto pass the time. But I feel like I should be doing interacting with her other kids, her child, and doing things around the house. But this month I've been ask to work four extra shifts last minute. I turned two down. But I have finally started saying if she needs me additional hours, I'm going to have to also work on homework... So tomorrow I'm now working 3.5 extra hours I plan to use at least that much time on school work. Honestly I try an plan my school work as much as possible around my schedule. But I know I can't put off my school responsibilities too.