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
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Reading is very common at Hogwarts and the wider wizarding world. Hogwarts has an extensive library, and most of their communications and news media are done over written mediums.
Contrast that to maths where the hardest thing most wizards would realistically have to do is basic money operations. Made even harder by their bonkers monetary system.