"Kiritsu - rei - chakuseki
(Stand - bow - sit down)"
As most doors in Japan were traditionally sliding doors, it's common for them to be used in the entrance to Japanese classrooms too. As the teacher enters the class, the clattering sound of the sliding door prompts students to take their seats hastily and get ready for the lesson. It's also standard for students to insert a board eraser in the gap above the sliding door as a practical joke.