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
Although that could also be them being perplexed by the fact that muggles did it with no magic at all.
Imagine us discovering a species that developed computers and tech just like ours, but using neither transistors nor electricity.