Is my coach a scammer?

I was trying to sell it off to buy the latest iphone, and my coach came up to me and told me that she wanted to buy my phone. My phone was brand new, I've used it for only a month and the price I suggested was way cheaper than the initial price. So she took the phone from me and told me she was going to pay it later in the evening. Evening came, she paid me LESS than 1/4 of the suggested price. She told me before this that she was going to pay the full price to me and now she wanted to pay it monthly after she got the phone. So I called her and told her that I want my phone back and I'm not willing to sell it to her since it was not as we bargained. And two days passed, the phone is still with her. Mind you, my phone is super brand new; i rarely use it because I have a Samsung. I'm afraid that she's going to switch it with a fake iphone or scratch it. Is there a way to check the serial number of the iphone? What should I do?

Phones have serial numbers, did you save it?

this is the time to call in your parents for help. A phone call to the school should straighten this up.

No