Why is society divided? The rich, the poor, the farmers, the people who live in cities, the intellectuals, the middle class, the handicapped, the professional sports players, and then the races, the religions etc.
People are not equal, we categorize others according to their race, religion, wealth, interest, working place, age, physical appearance, health, intelligence and more. Everybody is different and we are not equal, we are so different that even the criteria upon which we base our stereotyping differ from people. These difference is one of the reasons why we have different groups and have a divided society.
The reason why high-schools (and most schools, from kindergarten to university) are divided is because schools themselves are a miniature representation of a society, where you have different people playing different roles. And if you look carefully inside those groups that make up the school you will see that there are hierarchies and maybe even smaller groups within.
If you want more than a half-assed response to such question then you should study sociology.
high schools are all divided the punks the drugies and the popular kids why are they divided ?????