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.
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Obviously, JK Rowling was not a tactical mastermind.
However in the universe she created, any powerful wizard worth a damn would simply keep protective shields up all the time, especially against such simple things like “fast objects hitting them”.
Protego seems to be doing a lot with barely any effort to cast. Fred&George even create permanently enchanted clothing with Protego on it.