The incident isnt recent so im not sure if this is exactly what was said and if such an interpretation can be put on this
This is the issue i.e. if it was a boy and a girl the response would be different as they have been told in school that this is not ok but its unclear about if it was a non sexually defined scenario in the lessons or if it was boy and girl scenario.
I do agree that its someone chancing their arm in the same way many diy and professional builders do . Im leaning towards the doing nothing idea
People, particularly young ones, get things wrong.
Notice, remember, put it down to experience, but what's to be gained from wailing and seeking retribution?
Let it go, - unless he does it again.