"Just doing his job" is second only to "disorderly conduct" on the list of make-it-up-as-you-go-along police work. It's not about "laws", it's about demonstrating and exerting authority (read: bullying the populace).
Libertarian clowns are trying to get mileage out of this story, but the real issues are parents having the right to see their own children at any time, and that cars are prioritized over pedestrians.
http://www.wate.com/story/24005228/cumberland-county-parent-releases-video-of-his-arrest-by-a-school-resource-officer
Cumberland County parent outraged by his arrest
A Cumberland County father didn't want to wait in a long line of traffic to pick his kids up from school. That led to a series of encounters last week at South Cumberland Elementary, and now the dad's facing charges.