Have everyone stand up and begin the game, for example you might start by saying "Simon says, hands on head" while placing your hands on your head.
The students should follow your instructions, quickly putting their hands on their heads. If they don't do it correctly or are just too slow then they are out of the game and should sit down (you might want to be lenient on students making mistakes during the first round).
Continue the game with those remaining, slowly increasing the speed in an effort to catch them out. At any point you can try and trick the students by, for example, saying "Simon says, hands on ears" while placing your hands on your shoulders. Anyone who puts their hands on their shoulders is out of the game.
It's a simple, fast activity that is great for improving listening skills and is especially fun for younger students who love moving around.