School leaders in New York City and nationwide are being forced to make difficult decisions about teachers this year. With budgets still reeling from the aftershocks of the recession, layoffs are unavoidable.

Reasonable people agree that teacher effectiveness can’t be ignored when deciding who stays and who goes. Firing great teachers because they happen to be low on the seniority totem pole is madness. So is paying teachers with no regard to success or failure in the classroom.

New York City should be applauded for working to bring more information about student learning into the teacher evaluation equation. It won’t be easy, though, and school officials should plan for some bumps on the road ahead.

Until recently, using student test scores to evaluate teacher performance was the third rail of education policy. Just three years ago, the United Federation of Teachers pushed through legislation in New York state creating a moratorium on using “student performance data” to make teacher tenure decisions.

Times have changed. Earlier this month, the leaders of the nation’s largest teachers union, the National Education Association, circulated a policy statement endorsing “valid, reliable, high quality standardized tests that provide meaningful information regarding student learning and growth” as one element of larger, multi-dimensional evaluation systems.

That’s the approach being taken in New York City, where improvement in student performance on the new tests will make up only 20 percent of a teacher’s evaluation. Regular state tests will be another 20 percent, with the balance coming from classroom observations and other measures.

If designed well, the “performance tasks” being developed by the city, which can include essay writing and complex problem-solving, will help students learn even as their learning is being gauged. New tests always take time to calibrate and refine. The city should be cautious in using initial test results for high-stakes personnel decisions.

New York, along with 41 other states, is also adopting the Common Core State Standards, which will be accompanied by sophisticated new tests that are currently being developed by two non-profit consortia with federal grants. The city should be careful not to duplicate that testing effort. But overall, the city's approach is a welcome development for students who deserve to be taught by educators with a demonstrated ability to help children learn.