I want to give a few students in each class an opportunity to show off their work. If you have something cool, please volunteer. 

You have a sub tomorrow. I'm asking that the sub run the class exactly like I do, but she is in charge, so please work hard and behave. 

NEW: how to end your game

Have a 6th background that says "you win" 

Have a sprite that pops up and says "you win"

Have your sprite say "you win"

An ending is not a requirement, but all good games have some way of letting you know that the game is over.