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
Science: wow that is a nice animal you have there; have you seen out Large Hadron Collider, it accelerates clouds of particles up to close to the limit of causality and slams them into specially designed targets. We do this to probe the secrets of the universe at the edges of imagination and plausibility.
Magic: I can kill people by pointing my stick at them…
Science: yeah, we have that also…how big do yours get? We have a problem where we went too far and now can destroy the world, but because everyone has these weapons we live in an uneasy tension called mutually assured destruction, so no one uses their most powerful weapons for fear that others will also.
Magic: I can make it rain when I want.
Science: ok now that would be useful, you have to teach me how to do that.