Epic fail! Mayor Bill de Blasio and Schools Chancellor Carmen Fariña got their first major test early Thursday as the proprietors of America's largest school system. And they blew it.

While the city's Sanitation Department was marshaling 1,300 salt spreaders and 1,900 snowplows to counter the massive, well-documented winter blast headed toward the city, de Blasio and Fariña thought it made sense to require 1.1 million schoolchildren to attend class.

So as the city's Office of Emergency Management was issuing a hazardous driving advisory, school honchos were sending kids, parents and about 75,000 teachers slipping and sliding toward the schoolhouse doors.

But why? Nothing de Blasio and Fariña said Thursday makes much sense.

De Blasio said the National Weather Service had a best-case prediction of 3 inches of snow on the ground by the time kids got to class. Maybe. But TV weatherman Al Roker -- suddenly America's chief whistleblower on sweating pols who blame bum forecasts when their planning goes awry -- says the snow hit just as forecast. Midtown had 9.5 inches by mid-morning.

De Blasio correctly said that city kids depend on the 860,000 meals the public schools serve up daily. But is that a reason to keep schools open in hazardous conditions? If so, what's going to happen next week when the schools close for five days during mid-winter recess?

The truth is that while politicians try to project strength and certitude, weather decisions are notoriously tricky. They can make a mockery of the best tacticians. Ask any former New York mayor. The risk is even worse when the mayor and his staff are newbies. De Blasio hinted at that problem Thursday even as he stuck to his weak excuses.

He said his decision-making process needed to be more flexible and transparent. Let's hope he learned his lesson.