*mrlovebucket absolutely. It sounds very much that op has taken these events direct from the child, which I find worrying. DH is a head teacher and the parents of the 3 children would have been informed. While I'm sure there's mumsnetters who will think this is overkill but it's actually standard practice now for good reason.
He is only 5 but old enough to understand boundaries with private areas. He was told to do it by another child and it may well have been giggles all round. However, although not a safeguarding issue from what op has said it would certainly be 'logged' and all parents contacted. Keeping him in for one break is a common protocol in punishment , it's highly unlikely this would be unsupervised. Many schools have open plan areas and may not have been aware he wasn't alone. If -on the other hand- he was alone this would not be acceptable , keeping him in for two days in a row would also not be an acceptable form of punishment unless there's more to the story than we know. I'm doubtful a teacher would have thrown his drawings away, they may well have been scribbles to get through the time!
As I've said , this would be something that all parents would be made aware of. I'm sure he didn't understand the enormity of what he did , children's understanding of right and wrong varies hence the importance of information passing onto parents. In DH school he would also have had a chat about personal boundaries direct with DS , his friend and the girl. I would be surprised if any of them where not aware of it being wrong but it clearly needs reinforcement with explanation.
I think OP should also put herself in the shoes of the girl's parents. Of course the child may have seen it as part of play and not been bothered, in which case she also needs to be made aware of personal boundaries. Unfortunately a similar situation happened with 1DD (now an adult) when she was 6. Over the next two years she had huge issues at school and around getting changed for PE. She ended up having counselling in school.
Op whilst of course you are very concerned about your son the bigger picture needs to be seen here, one that you have the full story from the school and not just your DS.