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
If you haven’t already, take a peek at Harry Potter and the Methods of Rationality. It’s fanfiction, but absolutely worth a read.
Oh man I totally forgot about that! Oh nice they have it in ebook and pdf formats now
Just a warning: this takes pretty much every bad FF trope you can think of and turns it to max. Awful reading.
I stopped reading it halfway through, and was too lazy to figure out why.
This explains it.
To add, the podcast is phenomenal. Definitely worth listening to as well