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 [email protected]
I think everyone hated alchemy not because of Snape alone, but because it involves math too. Scaling the resulting doze of a potion, balancing it’s effects, predicting chemi-magical reactions of ingredients - it’s all math. And Severus was portrayed just like an average techie teaching a class of humame arts students.