No, the kid's answer is not "just as right", it is the correct and expected answer. The teacher's answer is wrong and proof the teacher doesn't understand the question. The entire point of the question is understanding that fractions of a whole are relative to that whole and you can't directly compare fractions from different wholes like that. 5/6 > 4/6 doesn't mean Luis ate more pizza than Marty, it means Luis ate a larger share of his pizza than Marty ate out of his own.