In that cheerleaders case it says: "6 News did some digging and found that most video recordings are legal with or without consent. " So, what does that mean in this case of the school principal?
I think that if the camera was positioned so that it only viewed the sinks or other areas where no 'bathroom activity' would be going on there, then I would be perfectly OK with having the camera in the bathroom. I think that you have to cut the kid some slack for taking the camera, he is an 8th grader and probably didn't think through his actions fully before removing it. I give him props for standing up for what he believes in, and his mother too.