old boy network

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A presumed unacknowledged system of association between childhood friends (especially those at school or university together), used for mutual assistance or favouritism.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an exclusive informal network linking members of a social class or profession or organization in order to provide connections and information and favors (especially in business or politics)