Man, this similar thing happened to my brother. He was walking home with his friends when this guy jumped out and threatened to stab them if he didn't hand over his skateboard. Naturally, not wanting to risk getting stabbed, he hands it over. What he did next I recommend you do too. Afterwards, he files a police report and an investigator comes over to get the details. He said it wasn't likely he'd find it but that he'd try.
About two months later(after he had to buy a new one), he sees a kid in his school with the exact same skateboard and he asked where he got it. The kid said he bought it off this guy for $50. I can't remember if he told him right then and there that it was his and that it was stolen, but he did tell the principal, explaining the story with his friends that were with him when his deck got stolen and saying he had a police report against the board. Despite the kid denying it was rightfully my brother's propertau, he eventually got it back.
So in the end, my advice to you is to tell your principal the story and show the pic while also getting some authorative backup.