* not a teacher
"lemon" is most often used to describe cars, and it is very common in that regard. Using lemon to refer to any other defective item would probably be understood, though not common.
Wikipedia says "A lemon is a defective car that is found to have numerous or severe defects not readily apparent before its purchase. Any vehicle with these issues can be termed a 'lemon,' and, by extension, any product which has major flaws that render it unfit for its purpose can be described as a 'lemon'."
So it appears that using lemon to refer to any defective product is fine.