Błędnie: Although the class was waiting near the stuff room, the teacher didn't notice them. Poprawnie: Although the class was waiting near the staff room, the teacher didn't notice them.
"Staff" to 'personel, kadra, pracownicy'. Dlatego w przypadku szkoły "staff room' to pokój nauczycielski. Natomiast "stuff" to rzeczownik o wielu znaczeniach, np. 'rzeczy', 'śmieci', 'bzdury'.