“How much did you get for/on the math test?” Does this sound natural to ask about the grade? Which preposition is right here?

"on" is correct here. I would phrase it "What did you get on the math test?" or "How'd you do on the math test?", because "how much" is not quite right to ask about a grade, it's a rating out of 100 after all, not a number that's counting something.

Typically you would say “what did you get on the math test” as in, what grade. You could also ask “how many points did you get on the math test?”

I’d use “on” but would more likely ask “what did you get on the math test?”

"How much did you get" sounds strange when referring to a test. How did you do on the math test? How well did you do on the math test? What score did you get on the math test? (I can't comment on "grade" as its more of an american english thing)

"How much did you get for the math test" makes it sound like you're selling answer keys and your friend is asking how much you sold one for.

An etiquette note- in the US this is considered a personal question. You could ask a close friend or family member, or a teacher could ask their own student in some circumstances. School scores are legally protected information, similar to medical records. Different English-speaking countries have different standards about this, of course.

In the UK you could use 'for' when just referring to the subject. So 'what did you get for maths?', probably in the context of just having had a lot of exams. 'In' would work there as well, but sounds more old fashioned to me. People would understand the meaning if you asked how much you got for the maths test in that context, but asking how much you got for a thing usually suggests you've sold it for money eg, how much did you get for your car (which you sold).

What did you get,?

In the American dialect this makes sense. In English as spoken by English people it’s “maths”.

"What did you get on the test?" "What did you get for #5 (number five)?"

Others have answered that "on" is correct. "What did you get *for* the math test?" could mean a couple of things: * there are multiple versions of the math test, which one did you get? * you are selling a math test (either a copy, or the service of taking it), how much money did you receive for it? (Yes this is immoral)

What did you get on your math test?