My Kindergartner Who Has Received Nothing But Good Behavior Reports

My kindergartner who has received nothing but good behavior reports (green smiley faces) all year, has over the past two weeks been displaying some really disruptive behavior at school. Today it was screaming at the top of his lungs, yesterday, he told someone to shut their mouth. Monday he stomped his feet, knocked over a chair, screamed loud enough for the entire world to hear, and last week he was telling people to shut up, and calling people stupid. I tried spanking, I tried the corner (sitting in the corner, standing in the corner), I've taken away privileges (computer which is his TV. we don't have cable). He doesn't do any of this stuff at home. I always boast of how well-behaved my child is. Now, I have no idea what is going on or what to do about it.