A school exam has 23 students in 23 different classrooms. The students have no way to communicate with each other in any way from their rooms. There is another room, the play room, that has two switches on the wall (X and Y). The switches have on and off positions but they start in an unknown position.

Students are randomly taken to and from the play room one at a time. They MUST change the position of only ONE of the two switches each time they go to the room. At any point a student can yell out, "Every students has been here!" If the student is correct that all of the students have visited the play room, then they all pass the exam. If they aren't correct then they will be expelled from school.

Before they start they are given one planning session during which they can discuss a method to win the game. What method can they use to ensure they all pass?