I guess it is supposed to be a joke, or meme. But it is pretty unfunny. It is just stupid.

Same in Germany

No. The teacher did not have it wrong. Does not mean the student is right … Marty and Luis both had their own pizza. Marty had a big pizza and “only” managed to eat 4/6th of it. Luis had a small pizza, and “only” managed to eat 5/6th of his. If you want to give a nitpicking correct answer: a single pizza does not have (4 + 5)/6th pieces. x/6th implies the pizza(s) were divided into 6 parts … so: it can only be 2 pizzas.