That one of them stole it is pretty low but I think leaving it lying around was fairly silly.
Hopefully you've learned a valuable lesson without too much or any damage being done.
It sounds like he didn't have a choice. It was drying on a rack at school along with many other photos. The OP stated that he doesn't have his own facilities.
Did you contact the police
I think the contest was unwittingly committing a criminal act by publishing your photo without your permission.
The person that sent in your photo must have signed a paper stating that this was his own work. Unless the contest director can show that to the police, I think that they are liable. And if they show it to the police, the police will know who submitted under your name.