Stupid Teacher!

I am so pissed for my 5th grader. He is a straight A student (yes, I'm bragging!) and he is a good kid with no problems to speak of. His conduct has been all E's and S's (Excellent and Satisfactory) all year and we have never known him to act any other way. Yesterday he came home with his report card with an N in conduct (needs improvement) in Social Studies for not turning homework in. I talked to the teacher and she said that he only turned in 1 of the 3 homework assignments that were being graded for the 6 weeks. When there is a child that has a history of good behavior, good work habits and straight A's, why wouldn't a teacher talk to the child or call the parents before it affects their report card? I'm waiting for him to come home so I can see where these assignments are but I'm still ticked at the teacher for not calling it to our attention earlier. He still got an A in her class but it's just the point that there was a problem and she never said anything about it. UGH!