There is no denying the bonds and the experiences of elementary school change the way we view the world and ourselves. The introduction of other family constructs and such concepts of responsibility, self-help and teamwork help us form some identity characteristics we carry into adulthood. Not all of those experiences are truly positive.
Although we are part of other groups before school begins, especially family and social activities like preschool or religious functions, school plays a very large role in how we identify ourselves. It is not wholly based on the people we encounter.