Children are always testing the limits of acceptable behavior. They, on a deep non-conscious level, want and desire limits... the feeling that someone is watching and guiding them with loving care.
Kids respond to different things, so I'm not a "beat them all" guy. But if sweet reason isn't working and the kid knows that the worst thing that will happen is an ineffectual note back to dithering, hand-wringing parents, then the teachers lose all authority with that kid. There's been a few in my fiancee's classroom that the best she could hope for is if they just slept through class.