Our school chemistry benches had gas taps, for Bunsens. If the class all (except the goody-goodies) blew air back down the taps, an gratifying whump! would emanate from the pipework when a Bunsen was finally persuaded to ignite (sometimes by an unwitting teacher), and the flame flashed back and set off the gas/air mix below.
Chris, you're safe from this trick. It was back in the days of low pressure town gas. I think natural gas is at too high a pressure and would tend to inflate your pupils (or students, if that's what they are called nowadays).
Andy