As a PT employee they cannot force you to work over your scheduled hours, and they can't schedule you for FT hours during peak unless you allow them to. In other words, they have to ask, but you don't have to do it, especially if you have another commitment like school or childcare etc. Of course, if you refuse, there might be payback, which means that extra hours opportunities won't be offered in the future. When I was PT, there was intense pressure to work 50 plus hours during peak, and then it was right back to just above minimums until volumes started to rise in the Spring. If you weren't totally flexible during peak, management would remember, and then one of their favorites would get extra hours. We have a couple part-timers who are in school and drive FO routes and then flex. Even when we have late freight at 1000 or so, they leave because of school and while management might not like it, there isn't anything they can really say.
If you don't need the money, tell them you have other commitments. There isn't anything they can do because the operational need/necessity rule doesn't apply to part-timers. Once you are FT, they own you. I will be right at 59.5 hrs for this week, and there is nothing I can do about it.