On Tuesday afternoon, a hundred or so parents of New York City public-school children, most of them mothers from the leafier precincts of Brooklyn, gathered outside the Manhattan office of Gov. Andrew M. Cuomo to rally against the potential extinguishing of the so-called millionaires’ tax. With $1.4 billion for schools cut from the state budget last year, these parents wondered why the governor was so committed to helping the well-off become even better positioned. “Millionaire rhymes with fair and share,” one placard announced.