What does her therapist think is going on?
A typical child needs stability. They need consistency. They need rules.
Then you have children that need it all of the time. No letting down, no backing down, no wearing you down. They test as a way of making sure they are getting attention.
It is in a negative way, because they may not feel like when they do a good job, an expected job, an ok job, this does not give them the attention they crave.
In an adult we would consider this a "high maintenance" person. You know, the employee that does their job, but wants a lot of thanks and public recognition, even though they are doing what they are supposed to be doing because they ARE being paid to do this.
You need to come up with a plan and stick with it. Work with her Therapist about what this should look like.
This will need to include her father, The school, the Principal and her teachers. Let them know you are working on this problem, but there may be mornings where she is going to be at school, but it will include kicking and screaming. Find out what this will mean so you can back them up. Will she sit in the office until she calms down? Are you allowed to take her with a change of clothes, because she is not willing to change out of her pajamas?
Mom find your strength. She is begging for stability. She needs you to be the rock in her life. She needs to know when you tell her no, you mean it.
No matter how hard she fights, you, No matter how loud she yells, that you are so strong, you will not back down, because you are the Mom, you make the rules, you love her and you know she can handle it.
It could be that she is afraid of growing up and having to be responsible for herself. Perfectionist? Afraid of change? Sensory problems? Self control problems? Something Traumatic or a giant change that has overwhelmed her?
All kids go through changes. Many times, they are nervous about not knowing what to expect. As parents we need to not feed into it, even though, we are just as frightened as they are.
The louder she gets, the quieter you should speak with her. Get yourself ear plugs, lots of them. They will not block out sound, but they can take the edge off.
When she was a young child these are the things I did to reenforce my expectations.
I do not make threats. I make promises. If our daughter started yelling, she knew I was going to send her to her room to calm down. After a while she would just go to her room and calm down before she yelled.
If our daughter begged for something that I had already told her "no" about, she knew it would be a very, very long time, before she would be allowed to even bring up the subject.
The deal is that if she asked for something and I said no and she acted ok about it, I would thank her for not getting upset when I told her no.
"Thank you for understanding."
" I understand you are disappointed, but thank you for understanding."
" I can tell you are upset. But this is just not something you can do right now."
We learned to make compromises and I learned that if I gave her options or reasons, this worked for her.
"No we cannot invite a friend over, because I have a lot of house work I have to get done today. Maybe if you help me and we finish, there will be time for a friend to come over."
"No, you will not be able to to go to Sally's Birthday Party, because we are going to Grandmas, house that day. How about you invite Sally over another time, to play and have lunch next weekend?"
"No I will not buy that book for you, but you have some birthday money. Would you like me to pay for this book and you pay me back when we get home? "