Like Pachyderm says, there's not enough information for an outsider to make much of a comment. I would maybe have her father make an appointment to see her teacher, discuss his concerns with her (preferably off the record) so there would be an extra eye during the day, another adult the child might/could confide in if need be.

But if she's not saying, asking and supposing too pointed a questions is only "leading" and that's not good. Since it's her father asking and she has a good relationship with him, were I the child I would not have a whole lot of difficulty telling on my brother if he were being mean to me.

A backache after shoulder problems that you think/hope were caused on the trampoline sounds consistent enough for trampoline and I can't think of how a backache would be caused by abuse; I was physically abused and don't ever recall a backache. I could see having one's shoulder "grabbed" and that wrenching one's back but still, a single little round bruise doesn't seem consistent with that level of grabbing and pain. I don't know, maybe she's been complaining about aches and pains to her mother and getting the ridicule so when you all asked, she was afraid of more ridicule or feels bad about needing comforting or something.

It sounds complicated and might be harder for her to express herself about than her age enables her to, also. But I think it's all speculation until you get more information.