Amazing how much children can learn when they're not running around screaming. Being loud and obnoxious doesn't help their education but it also interferes with all the others in the classroom.
Well, here is the US classroom 'management' is a real issue.
First, off different kids come from different households, different expectations and norms.
Second, kids are exposed to different socialization experiences in preschool, day care, stay at home or in home day care settings.
Third, some kids seem to be less able to control themselves in a highly stuctured situation.
As a caveat, i was shocked when is started to work in schools, I am a child of the sixties and seventies. Even in a 'bad' school , the 'bad' kids showed more respect than they did today. Of course about a fifth of my graduating class dropped out senior years as well. And a 'race' riot occurred when i was in seventh grade.
Also the expectation for school nowadays are incredable. Kindergarten requires much more self control, social skills and academics skills then it used to. The day is very structured, expectations are high and you should already now your letters, numbers and be a pre-reader.
Now there are many factors that play into this and much of it has to do with self sorting by socio-economic status.
Expectations of behavior in house holds vary widely (especially for boys), in some house holds there is no 'horse play' allowed. In others shoving , hitting, taunting are the expectation. it really depends upon the people in the house and what they are like. Especially if mom is at work and older brothers and 'cousins' are around. These behaviors are a real draw back in a current kindergarten class.
Preschool, large well run day cares and some in home day care settings are very good at the social and academic skill building. Some are not, some in home day cares are basically a riot.
Academic pretraining varies a lot as well. In some houses the kids watch Sesame Street, they are read to and taught their letters, they are encouraged to do things like sit still for an hour while they watch a movie. In others they watch all sorts of TV (Jerry Springer), they are exposed to frequent swearing and confrontations and who knows what.
Then there are the kids who have none of the skills, they are not 'well socialized' they are used to being the little king or queen of the house, they have never seen a book, they have watched 'Training Day' and 'Scream' a number of times and they think that pro-wrestling in the height of culture. They are not used to having to be polite to others, get along with others, follow a schedule, they don't know their letters or numbers. And often they have different expectations than others. They can say cute little things like "Whats up nogger?", and "I'll beat you ass".
On top of this, there are the ones who really have emotional and other biological issues.
And now a days we don't just kick them out of schools and tell them not to come back. Which is better but frustrating. These issues pervade all of education.