You could, but brush (without toothpaste) *before* lunch. Why before, so you remove some of the bacteria that cause the bad breath before they get a chance to feed off your food.
In general it's a bad idea to brush your teeth immediately after eating (acids in food weakens the enamel, and you risk eroding it by brushing it when it's soft), should wait at least 30 minutes before brushing. If you don't have that half hour between lunch and next class, then don't.
Also after eating make sure to rinse with water even if you're not brushing, that will help too.