The modest gains in the third year of Common Core testing of New York's public school third- through eighth-graders could be heartening. Unfortunately, we have little idea what the results released Wednesday mean on Long Island, because so many students didn't take the tests.

Statewide, about 1.1 million kids were due to take the tests this past spring, but only about 80 percent did, as opposed to 93 percent in 2014. On Long Island in 2015, about 50 percent of students opted out.

In English, the percentage of test-takers scoring "proficient" or better in 2015 was up a bit to 31.3 percent from 30.6 percent last year. And in math, the gains were encouraging: 38.1 percent were proficient this year as opposed to 36.2 percent in 2014 and just 31.1 percent in 2013.

But now we know more about the kids who opted out. According to the state, they are more likely than others to have done poorly on standardized tests the previous year. Also, many parents probably held kids out because they test poorly.

As a result, the proficiency gains are undermined because the underperforming students were more likely to stay home. The results, particularly on Long Island, are going to be almost impossible to use in many districts and classrooms, and that's a real problem.

There are good reasons to be wary of standardized testing. There may be too much of it. The tests are imperfect. Teachers evaluated on the results of kids' tests may tend toward teaching to those tests, or pass on their anxieties to students.

No test is perfect, but letting students opt out means parents are denied a marker of progress. Opting out actually works against attempts to make exams better because the tests are harder to evaluate for grade-appropriateness and clarity. And teachers should be judged partly on the success of their efforts. The nationwide movement to make that happen isn't going away, no matter how much teachers unions fight it.

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Legitimate concerns are being addressed. Pearson Education, the loathed supplier of New York's tests, lost its contract. The State Legislature is now requiring that Common Core standards and testing be evaluated continuously by education experts. And changes in the teacher evaluation process mean no one can be penalized unless he or she does poorly on both the student testing portion and the classroom observation portion. Bad test results alone can't sink an educator.

There is a federal requirement that 95 percent of eligible students take the tests. The federal and state governments could penalize our schools financially, but probably won't -- not yet. But they will demand action plans from districts to reduce opt-out numbers, so we can see how students, teachers, schools and districts are progressing.

The test results this year did tell us one thing: They mirrored the performance of New York's students on international exams, indicating our new proficiency standards are the right ones. The divisiveness of the past year did not advance the goals of making sure our students get the quality education that taxpayers generously fund, and of preparing them for further education and careers in the global marketplace. To let students continue to opt out of the tests would be to opt out of reality.