This game is popular with all primary school classes; make the story more or less gruesome according to age. You need a small glockenspiel or similar, and a way to prevent the children actually seeing the note you are playing.
Line the children up across the back wall of the hall, or on the far side of the playground, facing you. Start the story:
“Once upon a time there was a village full of children, who wanted to come to collect their Easter Eggs/Christmas presents/go to the Beach/ (choose according to the season, add more destinations as your imagination suggests), but between the village and the Easter Bunny/Father Christmas/Seaside there was a deep, dangerous, bottomless, gloopy, stinky, slimy Swamp (this is where you modify the description to avoid younger children having nightmares. I speak from experience.)
“However, this deep, dangerous, bottomless, gloopy, stinky, slimy Swamp (children like the repetition) was a Magic Swamp, and at certain times, it was possible to take One Step without sinking in to the deep, dangerous, bottomless, gloopy, stinky, slimy Swamp (and all the repetition makes the activity last longer).
To cut a long story short, the game is that the children may take One Step when you play a specific high note, but if you play a low note, it is not safe to move, and if they do so they will be “glupped” – sink to the bottom of the gloopy swamp, or in other less scary words, have to sit down.
“When you hear this note – ding – it is safe to take One Step. But if you hear this note – dong – then you must stay still, or be glupped for ever.”
To make the challenge easy, you can choose, say, top C for safe, and low C for swamp. You can increase the difficulty by choosing notes that are closer together, or by having top C for safe, and all other notes are swamp notes, or whatever seems good to you.