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Managing the city has never been among Mayor Bill de Blasio’s strengths, but boy is his fecklessness showing amid the COVID-19 crisis.

On Sunday, he finally shut city schools. Yet it’s now clear that City Hall and the Department of Education didn’t prepare for it.

De Blasio says he wants kids to begin online learning within a week, but the DOE is only now training teachers in how to do their work remotely. About a third of the kids don’t even have home devices, so officials are scrambling to provide them.

“We’re literally building the plane and flying it,” admits Chancellor Richard Carranza. What were all those City Hall “desktop exercises” about, then?

And, sorry, the mayor’s claim that it might be impossible to reopen schools even if the state gives the OK in a few weeks is just ridiculous.

Instead of making the rounds to every news show that would take him (not to mention his gym time), the mayor should’ve been figuring out these contingency plans — and cracking the whip over Carranza and every other agency head to likewise prepare.

Which doesn’t mean micromanaging to the point of paralyzing other decision-makers, as The Post reports de Blasio has also been doing. And, worse, preventing other leaders from correcting his misinformation.

He’s been getting “a lot of things” wrong in his public comments, one source said — such as about who needs to be quarantined and how the virus is spread. The logjam he’s created has left his relations with Health Commissioner Oxiris Barbot “toxic.”

It’s like he’s trying to show the very opposite of leadership.