...I have also have seen a LOT of assumptions made and a lot of accusations. I do not thank you for those. Those of you making these accusations are assuming that I just hate Suzie and wish she wasn't my gymnast to deal with. That is not at all the case.
Just know that kids know when you don't want them there, they do. You did say you didn't want her in the group right away and brought it up at four staff meetings. You didn't say you tried for three months to help her build her strength and then decided you didn't want her in a group. You said she made you want to pull your hair out in the first few weeks. To me that said young coach/employee, needs to learn patience when crap like this lands on her lap (which someone else put there). It will happen again. It's an opportunity to lose or build credibility with your boss and coworkers every time.
Lots of my advice was actually trying to nudge you toward trying some new strategies with your bosses/coworkers. I have had years of being on and heading up teams in a work environment, and if you read back through my advice much of it was geared toward you as an employee not getting what you wanted from your boss, and trying to come up with some new tactics. I am not an "owner" so I can't function as dictator, it's always been as part of a team.
There is someone at your place of employment who put her on that group that you have called out publicly repeatedly by bringing it up over and over at staff meetings. Everyone hears everything at work that happends in staff meetings.
We can learn from what happens when we get in a tough spot at work, or not.....that's up to all of us when we get in these situations.. On the rare occasion you can change other people it always means the benefit of changing is more than the cost of not changing for them. They don't change because you prove how right you are by documenting the crap out of everything, it does not work that way in places of employment. You can on occasion force someone's hand that way, but you are taking a big gamble by "winning" that way and it might cost you. That's not to say not to document, you should , but that is not the way to influence people.
People only change when there is some benefit to them. So why isn't your boss taking action? What is his cost versus his benefit in backing you up on this one? It's going to happen again in some way or form because there are always a few in the bunch at work that or going to make things prickly. I did think to myself...hmmm as your boss. You keep calling out this situation about a particular kid in staff meetings over and over and it's not working. You are not getting the action and backup you need from your boss. Why? What can you do to be more effective. Heck, or you can just think I'm an idiot, you that you did everything right, this whole thing is the other person's fault, and carry on...