Our education system has this funny quirk of grouping kids by birth date — rather than, say, intellectual ability or achievement or interest.

But developmental pathways are as individual as kids themselves.

And so there's a perpetual back-and-forth about whether to put certain kids in school a grade behind or ahead of their actual age.

Recently we covered the research on "redshirting," or the practice of starting kindergarten a little late. That researcher concluded that it's usually better to go ahead and enroll kids as soon as they're old enough. For one thing, they will earn more money on average over a lifetime with that head start into the workforce.

Now comes a big study to say something different: Holding kids back at third grade when they don't meet the academic standards will give them a boost in achievement, by some measures. And, it doesn't affect their likelihood of finishing high school.

Nationwide, about one in 10 children must repeat at least one grade, and they tend to be disproportionately low-income or come from minority groups. Following on the stereotype of the too-tall kid crammed into a too-small desk in the back row, previous research has looked at the potential downsides of grade retention: stigma, peer group mismatch and reduced expectations from teachers.

This study adds a counterbalance: If students get proper help and attention, they may be able to improve on low performance, given a little extra time.

Guido Schwerdt of the University of Konstanz in Germany, Martin West of Harvard, and Marcus Winters of Boston University, followed the fates of almost a million Florida students who were in third grade in 2002-2003. That year, in the wake of the federal No Child Left Behind law, the state put in a policy that all students who missed the mark on the third-grade state reading test would be automatically left back and given remediation. A total of 16 states plus the District of Columbia have such a rule.

The authors compared Florida students who just passed the reading test with those who just missed it and were held back. They looked at their test scores for eight more school years in reading and six more in math, plus high school transcripts and graduation figures.

The students who were held back did better in the first few years. Over time, the effects faded, but they still had higher GPAs and took fewer remedial courses in high school.

Ultimately, though, there was no discernible impact on high school graduation rates. This is significant, because being overage for one's grade has long been considered in itself a risk factor for dropping out.

As Harvard's Martin West notes, "the results are mixed – and therefore disappointing for the policy's proponents, especially since holding students back is costly." On the other hand, he adds, the worst fears of critics were not borne out by the results.

It's important to note here that not every student who scored too low was held back. English language learners and special education students, for example, were generally promoted anyway, and teachers have discretion to promote general education students who they think can succeed in the next grade.

West says what he really wonders about is why the left-back students, who make demonstrable academic gains, still do no better when it comes to high school graduation. "The big question for me is what happened in high school such that better preparation did not translate into improved outcomes."

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