How do you think work overtime without pay?

that's sucks of course..but it happens anywhere wingain. in so many countries including mine, we're asked to work overtime but no extra payment for that. yet, they still don't give us choice to say no. i'm working as an instructor with no current schedules. whenever i have class, i'm working but if not i stay at home. they won't cut my payment eventhough i have no class at all for a month, but they also won't give me an extra payments if i have a crowded class in a month. sighhhhh...