He was abducted by Hagrid when he turned 11 so that would place him maybe around the fifth or sixth grade.

I don’t know if canonically there are math classes at Hogwarts.

The thought came to while I was watching the anime Mashle. If you are into Harry Potter and One-Punch Man I’d recommend giving it a watch.

Someone mentioned this community below; I wanted to highlight it.

Small promotion for !harrypotter@literature.cafe