Rent-a-cops only have very limited authority wherever they're employed, but there is truth in the article that there has been and still is a continuing rise of this sort thing happening all too often now, with police and rent-a-cops overstepping their bounds, using far more than needed force to apprehend people.
IMHO, what the parents of that girl need to do is take them to court for this assault, which is exactly what this is, assault.
Seek punitive damages, the immediate termination of all those rent-a-cops involved, as well as sue the principal, the school, and the school district.
Hell, that principal should also be fired.