Don't eat/drink acidic foods like tea with lemon or anything that might promote acid reflux. No caffeine, no booze, no ciggies.
Stay well hydrated, use steam if you're in a dry climate. And if you do start to lose your voice, stop talking! Don't even whisper. Try not to talk unless you have to - your cords will recover faster.
The best way to avoid laryngitis is to not catch the cold in the first place but I know being a teacher means being on the front line of every cold and flu season. There's only so much antibacterial soap in the world.
Good luck, I have several teachers in my family and different colds have already swept through twice. This last one snagged five people so far, it must be a new viral strain, no one seems to have any immunity and it's highly communicable. Since everyone else is sick, I'm practically in a bunker. I've got orange juice, anti-viral Kleenex and Purell, I'm waiting this one out.