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
Plot twist: jk is a selfhating queer and her subconsciousness wrote Harry Potter to break free without disappointing her self hating parents.
That’s an interesting theory. He literally comes out of a closet, after all.