Not every single idea is misguided in Mayor Bill de Blasio's plan to save -- rather than shut down -- 94 deeply troubled city schools.

The mayor wants to lay out $154 million over the next two years to give students in these schools an extra hour of daily class time and to fortify teacher training.

And he will designate these institutions as "community schools" -- providing students in need with mental health services, enhanced nutrition programs and other benefits the way some charter schools do.

Similar services are needed in hundreds of city schools.

But they don't add up to an effective strategy for turning around crisis-ridden schools that for years have failed to teach. Deeply troubled, chronically failing schools should be closed now -- before they do more harm. New schools must open to take their place. Some things to remember:

You can't believe everything you hear. Cincinnati, which pioneered community schools, has been showered with praise for its program. De Blasio has said that plan has "endless potential." But independent analyses show that while some Cincinnati community schools have improved over time, the academic performance in others remains dismal.

A fresh start often works. As he rolled out his plan this week, de Blasio indicated that his predecessor, Michael Bloomberg, just gave up on failing schools. Wrong. While Bloomberg closed 200 schools, he opened many more -- increasing city schools by 50 percent. Most are smaller than the schools they replaced. Most are in poor areas. And most are showing impressive results -- with graduation rates up 9.4 percentage points and college enrollment rates up 8.4 percentage points.

This isn't about politics. New Yorkers often hate school closures -- even when the dysfunction is obvious. Why? Because it's hard to part with beloved neighborhood gathering places. And because education unions dread the accountability and disgrace that come with closings. But the mayor must resist those political realities and provide all pupils with effective schools without further delay.

Any plan without real accountability is certain to fail.