How can I keep my hair straight all day?

I have very wavy, shoulder length hair and I can straighten it and it looks really nice but as soon as I go outside my hair poofs up and then when i brush it its really curly, I have some shockwaves products and some wax and stuff like that but I cant go out now to get some stuff because I need it straight for tomorrow because I want to look fab for my first day at school