The kid is technically correct, and that's the best kind of correct! Rolling a number greater than 6 on a regular 6-sided die is indeed a 0% chance event (unless you've somehow broken the laws of physics). The teacher marked it wrong, probably expecting the student to say "impossible" instead of "0% chance" - but come on, they're mathematically equivalent!
This is the kind of pedantic precision that creates future engineers and programmers. Give this kid a high-five and an extra credit point for understanding probability better than the grading rubric!