What sort of a structure/schedule/routine does she have for the day? I find with my students, that having a predictable and "safe" routine is the best way to go. Things can be changed around, but that routine and consistency are key. (I'm NOT saying that you don't have a routine nor am I saying that you're not consistent - I don't know anything about how your days are run is all).

Can you sit down with her at some point when she's alright, and work out a new schedule? Schedule in what time she's to be eating meals at, what time she is to take a shower, etc. Work in all of the daily routines. Also work in school lessons and activities in to the schedule, and spread it out and mix it in with free time.

If possible, you could make a big weekly board that has the different activites on and what time they're be done it. Put velcro on so that things can be changed around.

Say, if she's supposed to be reading from 2-2:30 and she decides to refuse to do her work? Well, then that reading square would be moved to her next "free time" slot. She needs to accept that she has responsibilities, but it can give her some control over when she does them. If she has it scheduled to be working on math from 10-10:30 and then decides that she would rather watch tv during that slot? That's fine. She can go and swap out whenever her tv slot is and do the math during that time instead.

It gives her some ownership, but also reinforces the fact that she does have things she needs to do. Point out to her that "tantrum" isn't on her schedule and that it would take up a "free time" slot so if she's hoping to spend time playing with her toys, that she'd best not waste it.

Then forgive things if it's only a few minutes.

Honestly, with my students? (They're in grade 4). With my behaviour kids, I do have a "relax" area where they can remove themselves from the activity and sit and chill/read. After they've been there for a bit, I go and sit with them and ask them what's up. Usually they'll tell me. If they don't then I leave them be for a bit. Sometimes they'll rejoin after talking to me and I invite them back. Sometimes they just return on their own. Regardless, they know that the work that they're not doing still needs to be done. I never excuse them from an assignment.

Sometimes just walking away works. Like "Julia, this isn't helping you. You still need to do that math work. I will talk to you later." and then walk out of the room and let her scream it out. When she's calmed down, then ask what made her so upset. A lot of the time it's frustrating or confusion or perfectionism over the work. Then give her a helping hand.