Too black and white, I am afraid.
The facts are not in question, he looked, he saw.
It is entirely down to intent... she has said she thinks he did it deliberately. He has said he did not.
I believe they both are sincere in what they say. In other words, I believe both of them... at the same time... equally. Cognitive dissonance in action.
You do not understand me, or the situation. And the Head is in the same place. Good student, good member of staff. Both are cooperating, telling the same story, both are very embarrassed but she believes he acted deliberately, he says he did not.
Now, without crawling into their heads, how do you decide, one against the other? You seem to want him to be guilty. I don't think he is.
Others might say 'ah well, she acted maliciously' - I don't think she did.
I am glad it is not my decision, as I would doing someone a disservice. One that, getting back to the OP, would have been less likely to occur had she been wearing uniform appropriate clothing!
Which was the point I was originally trying to make!