If this is your worst problem right now.... you're overreacting.
Sorry. But it sounds like you need to understand that in life, things don't always go as we plan. 15 hours over the course of a month or two is nothing. I worked 9 unplanned hours yesterday. That made my day 21 hours long.... It happens. It isn't the end of the world.
It sounds to me that because this is a class you aren't really interested in you don't feel you should have to put in extra work. Think of it this way. You graduate in May. Do the work, do what you have to so you can get by, and get out. Easy?
College isn't just to teach you specific knowledge. Life skills are as key to any of it as anything else. Being able to adjust to a small commitment over a few months shouldn't be this kind of problem. While I'm sure money is tight, you could have taken 1 or two days of work off and been done in just a few days. You say you work that schedule "most days". Well, that means "some days" you don't....
Time utilization and flexibility is no small thing... Between work and school it looks like you have about 42.5 hours of "filled" schedule, with weekends free? So you have time to study. There is no flexibility in your work? By your choice or your employers?