As for my predictions:
I think its the blonde chick who hit her to get her sent to the infirmary in the first place. For one how hard do you have to charge someone playing b-ball to send em to the hospital? From my years of playing basketball, pretty damn hard.
The friend seems too obvious and I feel if it was her she'd have had a different reaction when she saw her messed up jersey and made up an excuse to leave to that of "passive bystander," especially since the author was making a parallel to those who weren't directly bullying teach but didn't help him out.
Not to mention blondie was the one who brought up sending the text and not receiving a reply, which caused the rest of the girls to start shit talking FMC.