The Whizzing Penny

You will need:

Some UK 1p coins or small washers
A drinking straw
A magnet (even a fridge magnet may do the job)

What to do:

Hold the magnet out in front of your face, and hang a penny from it by its edge. If the penny doesn't stick, you've either got the worst magnet in the world, or a non-magnetic penny. Try another one of whichever seems more likely.
Now take another penny, and hang that from the first one. Join them by their edges, so you're making a chain of pennies dangling from the magnet. Depending on the strength of your magnet, you might be able to make a longer chain; keep going until the last penny only just clings on.
This is where your straw comes in. Very gently, blow through it at the last penny in the chain. Blow to one side, and you should be able to make the coin spin. As it spins faster and faster, you can blow harder and harder, and you should be able to make it whizz around at speeds that must be absolutely terrifying to all penny-kind.

What's going on:

The very last coin in the chain is barely held by the previous one, and they touch only at the tiny spot where their rims meet. As a result, there's only a very small amount of friction acting on the penny when it spins - the most important forces acting on it are you blowing at it, and it having to push the air out of the way so it can spin.

If you think about spinning a coin on a table, there's the same air resistance acting to slow it down, but also lots of friction as it slips and rolls over the tabletop. So the magnetically-suspended coin spins far, far faster, and for much, much longer.

I've no idea how much faster it spins. Three times? Six times? Eighteen times? If your school has a strobe lamp, you might be able to work it out - let me know if you manage it!