How can I prevent my hair from curling?

I own the Chi, and I have curly hair. My hair isn't THAT curly, it's just poofy. I straighten it every morning, and I put in a little bit of product called Hair Food in it, to make it shiny and less poofy and frizzy, but by the first hour of school it's already wavy and starting to get poof. HOW CAN I FIX THIS?