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
People take offense to using adjectives describing a group of people as a noun. For example “the black community” or “people who are gay” describe a subset (describe a portion of the overall community,) whereas “the blacks” or “the gays” describe a distinct set (and imply that group as an “other”)