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
Sure but anyone can win any fight if they have position and prep time that the other party doesn’t: under the same logic Voldy can easily kill any sniper so long as he knows there’s one (and in a war, there is one). Reminder, at least in the movies he doesn’t actually need to aim for Crucio, and he can manually and literally laser crush a castle-wide energy barrier .