Upon his re-election in 2006, then-New York City Mayor Michael Bloomberg and Schools Chancellor Joel Klein offered the free use of underutilized school facilities to a bumper crop of charter schools opening that year—including my first. Fueled by this policy, charter-school enrollment in the city grew from 11,000 to almost 70,000 by the end of Mr. Bloomberg’s second term in 2013, and my one school grew to 22.