OK then work on expulsion. Arrest? Come on! Our law enforcement has better things to do than to be working on unruly students. Anyone doing things much harsher than this in my day got EXPELLED, not ARRESTED.
You need to study up on the topic. This particular LEO works in the building. It is not an uncommon arrangement these days, especially in schools whose students come from the less civilized parts of town.
I cannot say whether he's paid by the school district or the police dept.; however, this sort of thing is precisely what he is paid to do.
It was the "school resource officer" that "placed the boy under arrest" until his mother arrived, which means the school official kept the boy in the principal's office until the mother arrived. The kid wasn't taken down to police headquarters and fingerprinted and booked on charges.