Mayor Bill de Blasio just folded like a cheap beach chair — twice in two days, flinching first on his vow to lay off 22,000 city workers, then on his promise to open schools on Sept. 10.

For weeks, the mayor stuck by his deadline, despite threats and fierce criticism from labor groups: “Unions will . . . say things sometimes in a very dramatic fashion,” he said. But “the mission is to take care of our kids.” That drew rare praise from us — and, no doubt, thousands of parents eager to have their kids back in class.

But, with the United Federation of Teachers on the verge of a strike vote, and the usually quiet principals’ union also firing harsh shots, Hizzoner blinked — announcing a deal with the unions to delay classroom openings to Sept. 21.