We do have to be careful about pre-judging children, either through what we happen to read in their files, or what other teachers have said, or based on superficial behavior we see in other classrooms. Sometimes a child will have had a particularly rough year with a certain teacher and she writes about all of his shortcomings in his file. Well, his difficulties may very well have been justified, either by some underlying family issue or even a personality or temperament clash between him and the teacher. I'm sure we've all had a year like that.
Now, if the new teacher this year reads that file, she can react two ways... she can assume he will be a problem for her, as well, and gear up for trouble. She may plan to treat him a certain way, anticipating the types of behaviors her colleague wrote about. She will wait for him to act out, assuming he will. When teachers expect a child to act a certain way, he surely will live up to those expectations, and his chance for improvement or even feeling good about himself is lost.
Now, if she takes that file and puts it away, something different will happen. She will start the school year with higher expectations for him, giving him a clean start, on keel with the rest of his classmates. He won't be thwarted by the poison carryover from last year, but will be held to higher standards. And guess what, chances are he will rise to the occasion because if the teacher believes he's capable, he will believe it, too.
This is all about the dynamic (and almost magical!) relationship between the teacher's expectations and achievement motivation. I have seen it work in some of the most desperate situations, reinforcing my conviction that every child deserves to have someone who believes in her. And if she does, something wonderful is bound to happen.