A landmark report by StudentsFirstNY just tore the mask off the supposed “success” of many city high schools: They’re graduating kids without actually teaching them.

Mayor de Blasio touts a 70 percent graduation rate as proof his policies are working.

Problem is, half the grads aren’t ready for college (or a career in the workforce). CUNY deemed only 35 percent of New York City high-school graduates college-ready last year.

And some schools seem to specialize in awarding worthless diplomas to young men and women. The SFNY report flagged 65 schools with above-average graduation — but college/career-readiness rates 50 points lower.

The HS for Medical Professions boasts that 95 percent of students graduate — yet only 15 percent are college-ready. The UFT Charter School sports a 78 percent graduation rate, but only 11 percent are college-ready.

Chancellor Carmen Fariña just spoke at the Academy for Young Writers, whose graduation rate is 75 percent, yet with a college-readiness rate of just 6 percent.

In short, the problem goes far beyond the grade-fixing and bogus credit-recovery programs The Post has exposed in recent years. It’s a citywide plague of diploma mills — many of which face no pressure to improve.

Of the 10 high schools with the city’s worst college-readiness rates, only two are in de Blasio’s Renewal program for failing schools.

CUNY Chancellor James Miliken calls the lack of college readiness his system’s top challenge. It’s a nightmare for the kids, too: They have to waste their first year (or more) in college on catch-up classes.

We don’t blame de Blasio or Fariña or even the United Federation of Teachers for all the system’s failings.

No, we blame them for their fight against reform, and all their lies and spin as they pretend everything’s just fine.