Op, you can suggest they watch movies if you want, but it's a bit miserable expecting children to sit still and silently during their lunch hour.
You're being precious, you really are. The school supervise wet play as per their school policy, and the vast majority of children are able to comply without incident. A small number of children, including yours, take advantage of the situation and behave badly. For that, and being unlucky enough to get caught, they are punished.
If he was playing quietly in his room and you popped your head round the door to find him doing something he shouldn't, something he knew you wouldn't be happy about, would you think it was your fault for minimal supervision?
I teach and I honestly believe that the nicest kids, the ones who slip up but learn from it, have parents who deal with bad behaviour properly and don't make excuses for them : that'll come back to bite you on the arse when a 16yo ds is arguing the toss against your rules.