It would be a mistake to withhold more than $200 million in federal and state money Long Island’s school districts forfeited last spring because of boycotts of mandatory standardized tests. But it would be an even graver mistake to not make it clear to educators, parents and taxpayers that it’s a one-time pass. If participation rates this spring don’t reach the 95 percent mandated by law, the money will be gone.

Last week, leading district superintendents in Nassau and Suffolk counties wrote a letter to Acting U.S. Education Secretary John B. King Jr. warning him of a “tremendous impact” if Long Island schools lose the money. On Long Island, more than 40 percent of third- through eighth-graders opted out of the relatively new English and math tests based on Common Core standards. King, New York’s commissioner of education until early last year, knows exactly what the impact of taking those funds would be, and what turmoil has roiled the state’s schools.

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The rollout of Common Core was botched during King’s tenure in New York. The opt-out movement was fueled by many factors: a standoff over basing part of teacher evaluations on the tests, parent fears that classes were turning into test-prep sessions, anger at low scores on the harder exams, and a social media firestorm fed by teachers unions.

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Now all parties seem to be acting in good faith to make things better. Withholding hundreds of millions of dollars could jeopardize that. But the tests are important, too. They tell us not just how students and teachers are doing, but also how schools and districts are doing. So the U.S. Department of Education needs to be clear: A free pass on the 95 percent standard would be a one-time good-faith offer. From here on, if the students don’t take the tests, the schools and the taxpayers will pay the price. — The editorial board