Let me make sure I understand your POV, AD:
The school official makes an announcement in assembly the day before that throwing food in the lunchroom would be met with appropriate action.
The girls test that by throwing food, correct?
And they're suspended as a result, correct?
While I'd agree with you that charging the miscreants with an adult offense "throwing missiles" is over the top, I absolutely have no problem with the girls being suspended.
Kids test authority all the time. And it's disturbing how authority caves in to those tests, which probably explains a great deal of how many youngsters struggle with authority long after the age when they should have woken up and understood why things are the way they are.
I believe that process is called "maturation."
These girls will remember this little incident. And I'd be very surprised to hear that they again throw food in the lunchroom.