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
Around that age kids learn what negative numbers are. I feel like some of it might be important
According to the British government 5th grade math includes interpret negative numbers in context, count forwards and backwards with positive and negative whole numbers, including through 0.