old boy network

noun

1. an exclusive network that links members of a profession, social class, or organization or the alumni of a particular school through which the individuals assist one another in business, politics, etc.

noun

1. British informal the appointment to power of former pupils of the same small group of public schools or universities

A set of relationships based on past friendship or acquaintance that sometimes replaces or undermines official organizations: “He didn’t deserve that promotion; he got it because of his connections to the old boy network.”