Children with bad manners. Where do they get manners? From their parents. Take a look at the parent and how they act/react to their child and you'll get a better idea of why they act the way they do.
Of course, there are always verifiable diagnoses that affect the way a child behaves. But all things being equal, there are a lot of out of control children. I know because I spent the last nine years dealing with some of the most challenging students as a teacher and then as a Behavioral Intervention Specialist.
There's more to behavior than just meds and manners, including factors at home beyond the control of the child (not enough food, inadequate resources, parental discord, absentee parents, etc.). If the parent has a negative opinion of teachers/ school in general, that will be reflected in the behavior of the child as well.
It's hard to tell parents how to raise their kids, but I sure do enjoy trying. Often, they need an example to give them the perspective of the school. Unfortunately, this is medicine most aren't ready to swallow. Raising your child to respect adults and authority figures is a big step in the right direction.