I would say that teachers do like to hear the truth. They are going to hear anyway from their co-workers. Try to have something positive in your notes. (If most of the class misbehaves, and you have a handful being good, make sure you praise them.) The teacher does not want to hear that they have the class from heck (even if they do), they will take steps to ensure that it doesn't happen in the future. The trick is to keep them busy and on track.