Old boy network

An old boy network or society can refer to social and business associations among former pupils of top male-only public schools (independent secondary schools) in the United Kingdom, such as Eton, Harrow, Winchester and Charterhouse, private schools in Canada, and, to a lesser degree, to university students (notably Oxbridge), and indirectly to preservation of social elites over time.
See more at Wikipedia.org...

Noun
1. 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); "professional women have developed an old boy network of their own"
(hypernym) network, web