As presented, this is clearly the story of a mugging on school property. If it were my daughter, however,
I'd want to know the details of the situation. Did the girl who dropped them even know that she had dropped them at the time they got stepped on? Did the girl drop them and was trying to pick them up as your daughter just plowed on by? Was it that they fell right as your daughter walked by and there was no time to do anything about it?
Depending on the details of how it happened, I might have my daughter pay for them or I might call the administration and report the extortion.