[stigmata]

Suck them in with something like "xxx won the largest ever pot in the history of poker, worth $1000000. In 10 minutes, your going to understand the theory behind the audacious move he did to achieve this".

This will keep them interested while you go through the boring part of explaining the rules and (very) basic strategy. Not sure about the class size, but it might be worth bringing along some chips/cards to play a couple of practise rounds (whilst you continue talking/directing the game). Audience participation is very good for keeping people interested, even if it is only a few people.