Answer: PLEASE DO NOT LEAVE YOUR CLOTHES IN THE LOCKER ROOM OVERNIGHT.
Explanation:
Overnight is written as one word, when it refers to the duration of the night (as in this case) or when we describe an event that happens very quickly - we say that a person changed overnight, which means that he/she changed his/her personality unexpectedly.
The correct answer would be: PLEASE DO NOT LEAVE YOUR CLOTHES IN THE LOCKER ROOM OVERNIGHT.
