I want to offer a take on the other side of the story. As a teacher for quite a few years, I have always asked the previous teacher to tell me about the students- of course listening to the laws of lashon hara. I think it really gave me insight into who needed the extra attention especially at the beginning and who needed a bit of a firmer hand. I think i can honestly say that I never came in with a bad attitude toward a student based on previous reports. I just used the reports to make sure that everyone was getting the most out of the year- including the first few days that are oh so important to the years success!
I am the type of teacher who loves the challenging students who keep life exciting and are active and energetic in class, even if at times they go a bit beyond the limit of what is expected in a typical classroom.
I taught the last few years but its under ten years since I was in elementary school myself and I remember being a student quite well. Of course a teacher must keep a clear mind cuz kids can make major strides over the summer months, but I still feel that knowing the basics is important.