Happened at a below minimum wage job too. I was supposed to greet people, give them coupons and clean dishes.
When I noticed that my shift was almost over I asked if I should clean the dishes now. He said no. Alright fair enough.
Shortly before the end of my shift, I asked again. He said no again. I was a bit worried about how I was supposed to clean them in time but left again.
Then after my shift (we don't have a clock machine or anything, he has to sign a form so I could prove I worked my shift) he told me to clean the dishes. But he had already signed so I said my shift was over. He said “You just stay 30 minutes longer.” And even though it was scary (It was my first job, he was an older man and I was maybe 18 or something) I am glad I had the courage to say no and leave. I wouldn't have gotten paid for that extra 30 minutes and I tried to clean the dishes during my shift.