The next day, everyone noticed posters around the school. It told them about dancing lessons for the Yule Ball. Most people were confused, but Hermione, Harry and Ginny understood. For their final lesson (which was meant to be Potions) they made their way to an empty classroom, as directed by little signs hanging in mid-air.
In there, a huge vinyl was being set up by Filch the caretaker.
The students could see Professor McGonogall on the other side of the room. She beckoned them over, and then separated the boys from the girls and sat them on either side of the classroom.