That’s not middle school drama. That’s helicopter parent drama. An entirely different and worse thing.
Pony Club appears to do pretty good at teaching sportsmanship and team playing though I suppose it depends on how much the parents buy into good sportsmanship.
I have no idea on specifics but going forward it might be useful to think about having some good sportsmanahip/ anti bullying component built into your less program and into your orientation material for parents.
And for you personally to think through what you do when helicopter parents start to annoy you. I actually think it’s outrageous this mother called you about this.
The thing to be careful about is that sometimes kids are perfectly capable of negotiating their own social lives but their parents are sociopathic troublemakers. Also in a situation like this, the first person to contact you can make it seem like they are the victim, but for all you know the daughter of this woman is the antagonist.
Teen girls can get mean when they are competing for a limited resource, like adult attention and praise, or boys, or horse time.
Perhaps one part of team building is talking about how we all love and share the school horses, you will always remember them, when you move on to other horses and even your own horse. Try to get them past feeling too much ownership of a horse they don’t own and never will. Maybe talk a bit in passing about how much you care for the horses that you own. It’s really easy even for adults to start getting a possessive rescue mentality about the horses they lease or even lesson on. If you make a point of mentioning your ownership in passing every lesson in some context it might dampen this competition to “own” the horse. Like some women try to run off suspected competition by continually saying “my husband” this and that.
But if girls are fighting at school and online and bringing it to the barn, there’s not much you can do besides make them keep a lid on it at the barn.