It was not just left out, in my son's gym bag at school
Yes, that's unfortunate. That means that some scumbag thief already knew that your son had an iPad and probably saw him place it inside the gym bag (or maybe knew that he kept one inside the gym bag). The thief then took the opportunity to swipe it while no one was looking.
It really comes down to.... you just can't trust anyone (in public places). Sad but true.