I have been there and I know it is tough. This is what I did that helped.
I started slow and picked a tantrum prone event. Sounds like for you this could be music class.
I talked about how things have been going, how it isn't going well for anyone and we came up with alternate solutions. For music maybe something like her letting the teacher know she is frustrated, asking to get a drink of water and then coming back to work on the piece part by part.
Then we role played it out with stuffed animals. I got to be the screaming flopping kid and he got to be the grown up. We did it the old way with the tantrum and the new way with the alternative. This was very light hearted and he thought it was pretty funny.
Before we went into the situation, we reviewed the game plan and I promised a little reward if he was successful.
Once he was successful in one area, we moved to other ones.
Good luck!
