Wow, this has got to be really frustrating. And yes, there are things that really can be done that will help.
First, you need to realize that to punish a child of this age for something she has done hours ago will not work. They do not have the cognitive ability to link the punishment to something that happened hours ago.
All experts say that to change behavior, the modifying action has to be immediate and consistent. In other words, if the school, is not trying to modify her behavior - your chances are slim.
And that is one of the other problems. You are apparently only trying to change her behavior through punishment. What you need to do is to work on replacing her behaviors with more acceptable ones.
But, first, you really need to find out what is triggering her outbursts! If this is really only happening at school, then you must take some time off work and find out what is going on at school. Until, you know what is happening, it is difficult to know what to change.
You said she was tested for ADHD. For something to be ADHD, it must be present in two different locations and you said it only happens at school. HOWEVER, things like SPD - Sensory Perception Disorder or Autism can be more noticeably present in only one location. And these things should be looked into.
And it is very possible that her daycare places are the problem and you just have not found the right one?
So what to do? As I earlier said, you must visit the day care and observe her.
Second, you must teach her ways to deal with her anger and you need to find better ways to deal with her anger.
Essentially, the rules for behavior modification are that there must be immediate, short, consistent consequences. Do not expect overnight miracles. It has taken her awhile to get to this point and it will take a while to relearn control. But she will.
I would also look into buying "Cool down and work through anger" or "When I feel angry". This is part of a series of books aimed at 4 to 7 year olds and meant to be read to them at night (several times) and then practiced. Kids do need to be taught how to deal with anger. You do not try and use these techniques while she is screaming. But once she stops or later on in the day - you can refer back to them or pull the books back out.
You can find them here - http://www.amazon.com/Cool-Through-Anger-Learning-Along/dp/1575423464/ref=pd_sim_b_5
this is a good link on dealing with tantrums. While it is for adhd kids, the ideas are excellent.
Finally, here is an overall look at anger.
I hope this helps. Please feel free to ask questions.