Help! My employer wants me to work for free!?

So I work overnight for a very large international retail store (you guys know who it is) unloading freight from trucks. We are only allowed to work 40 hours (cheap s don't want to pay overtime). But my boss asks us to stay until the "job is done" which means that even though we only get paid to be there until 6 am he wants us to stay there until 7 am or even 8 am sometimes to get the job done. Now I would not mind working an extra hour or two a day but I would like to get paid for it. It seems so unfair for a poor college student to be trying to pay his way through school to get ripped off by a large corpoaration. I would say something but I'm scared of retaliation and losing my job. What should I do? Thank you all, I am in a very tight situation.