Help, parents won't feed me?

I'm 13. We have plenty of food in the house, but the refrigerator and pantry are locked and I only get about 1 meal's worth of food every day. If I bring anything home to eat, it get taken from me and I get punished. I'm overweight, and I used to get plenty of food but in the last year they have stopped giving me food. I'm fed around an apple and one half of a sandwich and something like a cheesestick every day. My grades are dropping rapidly. Help! What do I do? I can't get a job and I can't cook because I can't get in to the fridge. And our school does not offer pre cooked meals