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.
Everyone's parenting styles are different, typically because every child is different. Consulting a thousand different parenting books by a hundred different authors will tell you a million different ways to handle a child. However, there are some rules you see in action and just go, "Huh?"