Do it before and after school. They can't tell you off for that when it's outside school hours, and it's not illegal in the country to sell gum.
Tips on gathering customers: If you get a bus, offer it on the bus. It's a great way, as you're out of school, and most of the time the bus drivers don't care anyway (at least in my backwards county they don't).
You could start by selling it to people you know, and they in turn will tell their friends etc. I know it sounds like it won't happen, but it does work, as kids at my school do the same but with Mars bars.