Using the blu Tack or plasticine stick the closed sports bottle top over the hole in the CD.
Blow some air into the balloon and carefully stretch it over the bottle top trying to let as little air escape as possible.
When the bottle top is opened the hovercraft will gently hover accross your desk

How it works
Hovercrafts float on cushions of air.
The air from the balloon eliminates most of the friction between the CD and the work surface so that the hovercraft can travel further without slowing down.

Other ideas Let your pupils try gently pushing the hovercraft accross the desk without a balloon attached to show how friction between the CD and the desk will slow it down quickly.
Let them use different amounts of air in the balloons to see if that makes a difference.
Also let them try different sizes of balloon. Do bigger balloons always work better?
Does the balloon go further if the pop-up top is opened completely or only half-way?