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.

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