Fifteen thousand teachers.

That’s how many need to be let go for every $1 billion cut from school funding. And yesterday, Post State Editor Fredric U. Dicker reported that Gov. Cuomo plans to slice — yep — “$1 billion or more” in aid to the city.

How much of that will come from the schools isn’t clear.

But, obviously, a lot of teachers are going. The only question:Which ones?

State law requires City Hall to lay off the most recently hired teachers first, no matter how effective. And to keep the most senior, no matter how ineffective.

Obviously, that “last-in, first-out” rule would drain the city’s schools of some very good teachers.

Cuomo needs to push lawmakers to scrap “LIFO.”

And Mayor Bloomberg needs to pull out all the stops to convince Cuomo — and the Legislature — of that.

Albany’s aid chop might be far more than $1 billion: There’s been talk of as much as $4 billion in school-aid trims statewide; the city’s share of that, under existing formulas, could be $1.6 billion.

As noted, a $1 billion aid cut would translate into more than 15,000 teacher layoffs — and, interestingly, current seniority rules mean that none of the 1,300 teachers the Department of Education rated as unsatisfactory last year could be shown the door.

Clearly, many young men and women who’ve shown great promise in the classroom are at risk.

As are the pupils they teach.

Albany tends to ignore such inequities, opting instead to follow orders from its union paymasters — who insist on the seniority rule.

Which is why Bloomberg’s involvement is so critical. He’s staked his legacy on improving education; budget cuts that deplete the schools of good teachers could jeopardize all his gains.

There’s still a week to go before Cuomo presents his budget — and the city learns its fate.

But Bloomberg should be directing his efforts and resources now, to make sure any proposed trims in funding can be implemented sensibly.

Without ruining the schools.

That means convincing Albany to ditch the LIFO law — once and for all.