President Barack Obama is in danger of squandering one of his most important legacies -- better public education policies -- and doing real harm to our poorest students and America's future.

Over the weekend, the president and Secretary of Education Arne Duncan called for limiting the amount of time that students spend taking tests. It's an unfortunate and tragically oversimplified response to the challenges facing our schools -- challenges made plain by U.S. test results released Wednesday, which show that students have lost ground in math for the first time since 1990.

To improve education, parents and teachers need to know how well our students -- of all ethnic and racial backgrounds -- are learning. That's especially true in communities where schools are the best hope for escaping poverty. Parents deserve to know how schools compare with one another so that they can demand improvements where they are needed and hold schools accountable for delivering results. The same is true for principals and elected officials, to say nothing of taxpayers. That's why accurate information on student learning is so crucial -- and like it or not, high-quality testing is an essential element of that.

EditorialEditorial: Common Core testing gets an 'incomplete'

Until last weekend, the Obama administration had rightly emphasized the importance of using student-performance data to ensure accountability and drive school improvement. It has also supported governors and school superintendents who are raising the bar for student achievement, especially by creating and adopting Common Core standards that are tied to college and career readiness. Yet now that results from tests aligned to these standards are showing just how many students are not on track for college, the public backlash against the tests seems to have given Obama and Duncan a case of cold feet. That's deeply regrettable.

U.S. students once led the world in college graduation, but no longer, and our public-school students are in the middle of the pack, at the precise moment when people around the world are becoming ever more educated. The National Assessment of Educational Progress -- also known as "the nation's report card" -- has long made clear just how poorly American schools serve children. The latest report was released Wednesday, and it shows that in reading and math, only one-third of eighth-grade students are meeting the national standard.

Today, the biggest threat to American might is not any one country or terrorist group. It is our collective unwillingness to confront mediocrity in our schools. In the ultracompetitive global economy, the U.S. is facing a terrible mismatch between high-skill jobs and our labor pool. This problem will grow worse over time unless we expect more of ourselves and our students -- and hold everyone, including teachers and school leaders, accountable for success. Caving to pressure from union leaders and a vocal subset of parents who want to end testing and accountability will make it harder to achieve that success.

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It's true that some states and districts need to do a better job of coordinating tests, to prevent redundancy, and using them effectively. But that does not require an act of Congress. And the argument that teachers spend too much time "teaching to the test" misses the crucial point: When curriculum and instruction are aligned with high-quality tests, as they often are in Advanced Placement classes, classroom work will prepare students for success.

What's more, students will face tests throughout their life. They must learn to cope with the emotional stress that comes with the experience, particularly because many companies (including mine) use tests in hiring. I understand: Test-taking is no one's idea of fun, but it is part of life and shielding students from it does them a great disservice.

Using testing to examine a teacher's impact on student learning is also important. Studies show that the single best reform we can implement is to put an excellent teacher in every class. Leaving the lowest-performing teachers in front of students sets them back and hurts their career trajectory. Parents deserve transparent and objective data to assess teachers and schools. Just ask yourself: Would you want your child in a classroom with a teacher who helps students exceed expectations -- or one whose students consistently fall below expectations? Sadly, more often than not, poor children end up with teachers whom middle- and upper-income parents would refuse to accept for their children. One of the best ways to help identify such teachers is via regular data on student performance.

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If we want to give all our children the skills they need to succeed, there's no getting around testing -- and as bad as a federal limit would be, Congress has an even worse idea. Republican-sponsored legislation would require changes to state accountability systems that would give schools incentive to keep kids out of testing -- an odd and disgraceful instance of conservatives making common cause with the teachers union to impose a federal mandate on states.

The Common Core and other tough new standards adopted by states, along with the tests aligned to them, are exposing just how far our schools need to go. We ought to embrace that challenge, and make it a national mission to meet it in the near future.

The president and the secretary of education have made a grave mistake by reflexively seeking to limit testing. If Congress embraces their politically driven proposal, it will come at the expense of our most at-risk children -- making economic growth, along with social justice and racial progress, harder to achieve.

Michael Bloomberg, former New York City mayor, is founder and CEO of Bloomberg LP.