Open the bottle of diet coke and place in an open space.
Move the students back.
Pour the mentos into the tube, so that they are all lined up and will fall out fast. Put a card over the top of the tube, and invert it over the coke bottle. Pull out the card and move back fast. (Alternatively, use a paper funnel to pour in the mentos - the faster you can get them in, the better.)
A giant fountain of foam is produced.
The mentos have a very bumpy surface that provides a place for bubbles of carbon dioxide to come out of solution. It does so rapidly, generating a mass of bubbles which stay around long enough to form a foam that geysers out of the bottle.