Goodness me over reactions!!
I would be furious if DD was slapped too. However it happened when I was school among girls, and it will happen we are all kicking up the daisies. Hormones running high, the pressure boiler atmosphere of schools, etc all make teenage girls, as we know, volatile - its our jobs to try to teach them the best ways to deal with and voice their frustrations, and unfortunately some parents arent very good at that, and even the best parenting can still find your child snapping at others at some points.
The other girl has felt looked down upon because your DD didnt want to go to her house, she hasnt dealt with it well, school SHOULD be supporting her to deal with it better in future.
Exclusions are rubbish and do nothing except distance most kids even further than they were from the school system.
However - OP - I would say YABU to look for revenge on a mixed up and upset child (which is all that exclusion will effectively be, it wont teach her how to handle things better in future, therefore is of no use in providing protection for your own daughter, which is what you say you want) - YANBU to expect the school to keep in touch with you about whats happening. Call them back.