Thats crazy by my standards, how much time does school/uni require on top of that 40 hour week.
I've done 20 hours for my first few terms and that burnt my grades down a little.
I know its easier to talk the talk than walk it, sometimes we get caught between a rock and a hard place, If you think one side is going to crumble you need to act even in the face of risk.
Talk to your manager
arrange someone to cover the shift
know your rights and stand some ground
if there is even a couple of hours free in a day, arrange to meet someone who can sympathize and talk it through with
just a few ideas off the top of my head, they may not be practicable, but thoughts should be towards solutions and comprimises ^_^
good luck on the math!