I googled a bit and found this:
http://www.instructables.com/id/Layered-Soft-Drinks/Keeping sugar contents in mind I ordered DH to buy three colourful syrups (cherry, strawberry and something green I couldn't identify) which we called dragon blood, troll blood and snake blood, three types of juice (orange, pineapple and banana) which we called gold and silver juice and three types of soda which we called magic water, magic lemon water and magic ginger water.
He poured it all in Erlenmeyer flasks beforehand. I let two or three kids, which weren't busy with wandmaking or clasp decorating choose their three ingredients, then showed them how to layer them with the help of a spoon (In character as Professor McGonagall of course, including snippy comments and distributing house points!). The layering didn't work so well, maybe because we didn't have ice. The kids didn't mind, most of them loved whatever came out (interestingly, some were happy to drink dragons blood and silver juice, but others needed to know what stuff really was before mixing and drinking). Anyone who felt thirsty could come over and make another. It is amazing the amount of sugar kids can down in one glass without shuddering. For grownup tastes, I would recommend real fruit syrups as opposed to the sugar-food colouring mix DH bought, which may add a little tartness, and going to the trouble of adding ice. Well, or actually adding alcohol, which we did not feel was appropriate at the time!
Interestingly, DS and a few other kids mentioned the potions class was their favourite part. I thought for sure it would have been the treasure hunt through the fortifications vaults in the dark!
