Like everyone else, educators must occasionally miss work. They and their kids get the flu, too. Yet we also know that teachers are the single most powerful instrument that schools have to boost student learning. When they miss school, their pupils miss out on education. Indeed, research suggests that a 10-day increase in teacher absenteeism is associated with up to 25 days of student learning loss in math.

This is a travesty, especially for needy children and teens who look to their teachers not only to prepare them academically, but also to serve as supportive, caring adults in a fractured society that’s increasingly hard for them to navigate. Frankly, all of us should be concerned when teachers are missing a lot of time with our kids.

Our colleague David Griffith recently measured the extent of teacher absenteeism in public schools. He calculated the “chronic absenteeism” rates (teachers who miss more than ten days of school in a year) in both traditional and charter schools. He found that teachers in traditional district-operated schools are almost three times more likely to be chronically absent than charter school teachers. Nationally, 28 percent of traditional school teachers miss more than ten school days a year for illness and personal reasons—that’s on top of school holidays and vacations, as well as professional development days. In contrast, just 10 percent of charter teachers are chronically absent.

In fact, in thirty-five states with sizable charter sectors, teachers in traditional public schools are more likely to be chronically absent than teachers in charter schools. Hawaii’s gap of 56 percentage points is the largest; 23 percent of charter teachers in the Aloha State are chronically absent versus 79 percent of traditional public school teachers. In nine other jurisdictions (Colorado, Delaware, Florida, Idaho, New Hampshire, New York, Ohio, Oklahoma and the District of Columbia), traditional public school teachers are at least four times as likely to be chronically absent. The same pattern plays out when we examine the ten biggest cities in the country. Yet in top-performing charter networks such as Success Academy, Achievement First, and Uncommon Schools, chronic teacher absenteeism is virtually non-existent.

How so? Rampant teacher absenteeism in traditional public schools is at least partly attributable to generous leave policies enshrined in state laws and union contracts. On average, teachers get more than twelve sick and personal days per year, though only one-third of U.S. workers are entitled to ten or more sick days, even though non-teachers typically have a much longer working year (up to 60 days more).

Because they can’t easily be fired, thanks to tenure laws and ironclad contractual protections, traditional public school teachers can use all their leave days (and get paid for it) without worrying about what their principal and peers will think. Charter teachers—most of them working in the 88 percent of the charter sector that’s not unionized—don’t have that luxury.

American Federation of Teachers president Randi Weingarten has spent the past year blasting away at charter schools and other forms of school choice—throwing all manner of spurious claims their way. She knows that the best defense is a good offense, and wants to avert attention from union-initiated actions that negatively impact students. One of those is adult-favoring contractual provisions, such as overly-generous teacher attendance policies. These low-profile clauses have real implications for real kids—and do real harm.

Reasonable people may disagree about whether teachers, lacking valid cause for absence, should take all of the leave granted to them in their contracts.

What’s indefensible are union leaders who condemn charter schools while pushing for teacher contracts that put children’s interests last.