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 mean, there must be a reason why wizards are constantly perplexed by muggle technology.
The series would have ended much sooner if Harry had brought an AK47 to hogwarts
And why goblins have to do their accounting for them.