How to conserve iPhone cellular data?

I'm in high school and there's a new policy where your allowed to use your phone in school. Your only allowed to use it during class if your teacher tells you but during your free time you can use it whenever you want. I turn off my Wi-Fi because our school blocks almost everything even things such Nick Jr. And music and you have to go through this complicated process when registering. I turn off cellular data for all of my apps except for a couple that I use frequently such as Instagram, Snapchat, Pandora Radio, Safari, and a couple apps that use very little data. I still use so much data and I really try using less but I still use a lot

Asdf

Simple turn it off.