grammar - Why is it "less than 9" and not "fewer than 9" when we're talking about exam marks and scores?

This might sound like too elementary of a question but it suddenly popped into my head and started eating me. Why "I don't wanna be less than 90." and not "I don't wanna be fewer than 90."? Aren't marks countable? (90, by the way, is a mark out of 100 students get here at the end of a course.)

Thanks