I'm planning to steal an iPhone from school?

So, I'm 13 and I go to aCatholic private school in UK. So, I've been wanting an iPhone for a while now, but my family is too poor to afford it. On the other hand, there's this ***** at my school who has an iPhone 6S Plus, and she spreads rumors about me all the time that Have all made my school time a living hell, so what better than to choose her phone to take.
So, I hatched a plan. At sport, all the girls leave their bags in the changing rooms, with their phones in their bags. So, I decided I would go into the room for a drink or something, then go in her bag and take her phone. I'd take it home, reset it and take the Sim Card out so that it wouldn't be tracked.
I'm only worried that people are going to make connections and realize my new phone is that girls old one, or I'm scared the school or her parents will find out or track the phone.
I know I'm gonna get responses telling me not to, but honestly I just want help on how to do it and not get caught? So any help would be appreciated, thank you.

There's no way of doing it and not getting caught.

Just don't.

You could get into a lot of trouble you're caught. I don't care how much pain she's caused you, her family has paid for that phone and you have absolutely no right to take it. Grow up.

Stealing is morally and ethically wrong.

I'm not sure if there are free phone programs in the UK, but they have them in the USA for those who can't afford one because of low income. Look into that instead. Don't get yourself in all sorts of trouble over a stupid phone.

The sim card has nothing to do with tracking the phone. The phone has an imei number that stays with the phone even if you change the sim. As soon as they report to phone company that their phone was stolen, the phone will be locked and become unasable for anything other than an expensive ipod.

Um everyone will doubt you since you are the one going back into the changing rooms.

You donot.

Sorry but that is literally the dumbest idea I've ever heard. We went through that **** at 13 too with phones getting stolen but they always got caught, and it ended with the whole year having their bags searched, girls crying etc etc. That is a wrong thing to do and really childish and stupid. Seriously just grow up and be the bigger person.