Many of America’s elite colleges sit on massive pots of money, some worth billions of dollars. These endowments have grown significantly in recent years, the result of philanthropy and savvy investment techniques—and all the while they’re completely untaxed.That’s poised to change, though, if GOP lawmakers succeed in enacting their tax-overhaul bill, which is now inching closer to becoming law: The bill stipulates that certain higher-education institutions would, for the first time ever, have to pay a tax on the income from those assets.