Hormones.
She is a Tween.
Google "Tween development" online and lots of articles will come up.
I have a friend, that was a Director of a school... and she said that at this age & grade level, like clock-work, the girls, get SO hormonal and emotional and moody and so many issues. From about 11-12 years old, when they are pre-teens.
She said, there is also a lot of drama.... at this age.
They are still kids... and can't handle/cope with their emotions or issues... its a rough age.
But still, you can't tolerate that kind of behavior and it is disrupting EVERYONE and making the others cry. You gotta tell her parents. They must get this at home too, with her. TELL her parents, that she is making everyone cry... and they are miserable. Point blank. Tell them.
But yet, you are not "Mary Poppins" either nor the magical elixir for her.
What a doozy.
I would, just be matter of fact... and practical. Don't give in. Tell her in a respectful and firm way, that she is hurting everyone.. .and it is not tolerable. If she is upset... have her go to another room and cool off. She CANNOT hurt others. Period.
Tell her to be a decent human being.. to try her best. The screaming/crying won't help. And the BOSSY behavior is not allowed. Period.
If she does boss the others, tell them to step away from her and engage in other things... don't 'react' to her... just move away. Don't over rationalize to her or get too lengthy in "explanations"... just do quick efficient reactions... and stop. Let her scream/drama about it... but don't react. Just be matter of fact. She WILL deflate on her own... most kids do. But while IN the midst of a "tantrum" (which is what she is doing), you cannot 'reason' with a child. They won't listen.
So tell her, go vent, and go in another room.
I would just be matter of fact with her. Don't let her drag you into her drama. Because... NOTHING will be enough, for her. In this mood.
Tough love.
Some good books are:
1) "Have a New Kid By Friday"
2) "How to talk so kids will listen, and listen so kids will talk."
They have it on Amazon and even E-bay.
You are not her parent... and maybe just that fact alone, she will not listen to you. I don't know.
Maybe also, have activities that she can do by herself. Like reading, a craft, raking the leaves in the yard, just things SHE can do, by herself and feel proud about. JUST her own activity, by herself, so it is not something she can boss others about.
If that were me... I would tell the parents and not hold back. I do what I can that is in my ability and understanding for myself and my kids and the other kids.. . and for their well-being... and beyond that, the parents have to know.
It is also causing a lot of stress/anxiety for your kids... and the others... so something has to be done.
If she were like this in a school... they would be calling in the Parents.
all the best,
Susan