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
I reached out to him in private, I wasn’t going to copy our conversation as it is private.
I was very ready to move that community elsewhere, and I refrained from posting for a week before getting Gabe’s approval.