There’s a common, often unacknowledged reason private schools host Grandparents Day, at least in the Bay Area: The gray hairs are footing the bills. With yearly tuition ranging from about $6,000 to more than $29,000, and many families mortgaged to the gills, “going private” is often an option only in the most hypothetical sense. The Bank of Grandparents has been doing swift business with Bay Area families for decades. “Historically, grandparents or other relatives often paid tuition,” says Denise McCarthy, who directed Presidio Hill School during the 1970s and is now running for the San Francisco Board of Supervisors in District 3. “In the ’70s … private school tuition was expensive, relatively speaking,” she recalled. Even then, she says, her school celebrated a Grandparents Day and hit up the silvering community members for donations.