How to go faster on a snowboard?

There was a discussion in my cl today about speed while snowboarding. My teacher believes to gain speed you should remain with your board flat on the snow, while a student says it is basic principle that if you are on the edge of your board it will be faster. alot of clmates had different views on it I am just curious from a person who may actually snowboard or knows alot about it what is the correct answer?