I think the idea here is that the system is self-reinforcing. An initially undifferentiated mix of children self-segregates as the children begin to perceive difference, and privileged/norm-conformant children persecute unprivileged/norm-nonconformant children. The persecution has the function of excluding the unlucky children from the social opportunities that come from access to privileged society (both because they are actively blocked by the other children, and because they block themselves by not realising they can seek opportunities or not considering themselves capable or worthwhile). This exclusion compounds as the cohort ages, rapidly becoming total. The result is that only a small selection of the cohort has access to privileged channels of society, maintaining a relatively low degree of competition at that level.