When the facts don’t suit you . . . lie.

That’s how the city schools operate, a new study shows.

The StudentsFirstNY study cites “massive grade inflation” that serves to “conceal underperformance” by the schools. It accuses the city of “misleading parents by giving students high marks on school course­work even though the students are performing below grade level.”

The study looks at dozens of schools where nine out of 10 kids flunk state tests. Somehow, it notes, kids at these failure mills “pass” their courses — which makes it seem the schools are doing fine, when they’re not.

The study also blasts the city’s phony “school quality reports,” which show “the vast majority” of schools “meeting” or “exceeding” targets, even though at many only a small fraction of kids pass state exams.

And the deception’s getting worse: Of 100 schools the city deemed failures in 2014, only two got that label this year. Yet kids at the other 98 schools continue to flunk state tests. At 11, less than 10 percent of kids passed.

Why lie about the abysmal performance?

Because admitting it, the report rightly notes, would force City Hall to take steps the teachers union opposes — like closing lousy schools and making it easier for kids to escape to charters. And Mayor de Blasio won’t cross the union.

The new findings won’t shock readers of The Post, which has long reported on fraud and grade-fixing in city schools. Remember Melissa Mejia, who griped to The Post in August that she somehow passed a course despite not showing up, not doing homework and skipping the final?

Her high school wanted to give her a diploma even though she hadn’t met the requirements — just to goose its graduation rate.

Indeed, the citywide graduation rate is one big lie: While schools hand out diplomas to 68 percent of seniors, only 27 percent are deemed ready for college or a job. So why let them graduate?

As Mejia noted, the deceptions cheat kids of the education they’re owed.

The new report shows that the road to worthless diplomas is paved with years of worthless report cards.