Thank you for your advice.
Keep an eye on it. From your post it doesn't sound like this is a regular thing for him. Have there been similar incidents while he is at school?
Honestly, I can't tell you why he is doing this...there are several possibilities. The infrequency of the incidents suggest that at least at this point, there may not be a serious problem with this. If I were you, I would hold him accountable him though. i'm not talking about grounding or taking away privileges. but maybe make him clean the air pump, etc.
If it continues, maybe you should take him to the doctor just to make sure he doesn't have a bladder problem, etc.
Best of luck