It really is often not the student's fault when they're acting out. Crazy behavior in the classroom can so easily mirror crazy circumstances at home.
More often than not, teachers learn this lesson the hard way: through parent teacher conferences, phone calls, or other events in which they are unprepared to meet the source of crazy, erratic, or disrespectful behavior.
As a parent of 2 and an aunt to ... um ... like... nine (don't quote me on that number) I can assure you that kids are freaking creepy. They just are. It's not always bad creepy necessarily, but I swear the younger they are, the higher the creep factor.