How to keep my iphone cool in a locker?

I live in Arizona and school starts on the 5th of august but I just remembered that my dad wants me to bring my phone to school with me in case of an emergency. And my lockers r outdoors and it gets 100 plus degrees here everyday. I don't want my iphone yo over heat in my outdoor locker so any ideas on how to keep it cool?

Added (1). I have to keep my phone in my locker

Basically, I bought one of the ice packs from walmart. Those blue ones with water instead, I then froze them up in the freezer. When you put the phone in the locker, put some ice packs near it. Though, I usually have my phone in my pocket. I never keep it in a locker. If I want to keep my phone safe when not at home, then I'll put it in a pocket.