We educate students in groups. In addition to the obvious financial savings over individualized instruction, it makes sense for a social species to provide many opportunities for young people to interact with each other. Our K-12 school system thus provides 12,000 hours of non-kin social awareness and interaction that's embedded within the curriculum and classroom life.

Grouping students into class groups poses the problem of who should constitute a class. Students could be grouped by various criteria, but age currently dominates. Students thus begin Kindergarten at about five, and continue through the grades with students approximately their own age.

Students who are significantly more or less intellectually advanced than the age norm create problems, so separate gifted and special education classes exist. Students who create serious behavior problems are also often placed into special classes.

A shift towards broad inclusion has occurred in recent years, partially spurred by a national policy to encourage the racial and ethnic integration of schools. The rationale is that it's important for students in our very diverse society to become comfortable with social and cultural diversity. They thus should have daily access to as wide a range of the student population as possible. For example, placing a blind or wheelchair-bound student into a traditional classroom may create problems (just as racial integration initially created problems), but it's important for minority and handicapped students and the rest of the class to daily confront and solve such problems. The belief is that a more accepting adult society will emerge from such inclusive efforts.