I hope someone replies explaining to this kid that while yes they can be used in the singular in English that is only correct if we don’t know the sex of the person. The teacher is correct in that they is improper in this instance. All this confusion about they makes me wonder what is happening to education requirements to take a second language. Once you learn a language like French where even they is “gendered” it becomes quite clear how ridiculous using they to erase sex is.

OP is apparently German, so s/he is already presumably working with a language that has a built-in masculine/feminine structure.