As America’s public schools welcome back students from summer vacation, enrollment is projected to surpass 50 million students this year.

But many of these students are falling behind. The United States ranks No. 35 in the world when it comes to mathematics -- behind Russia and Vietnam. When it comes to science, the United States is pegged at No. 27 globally, trailing countries that most Americans couldn’t even identify on a map. Reading scores are the most disturbing: 64 percent of America’s eighth-graders read below their expected grade level. Among students from low-income backgrounds, roughly 80 percent of eighth-graders score below grade level in reading.

Yet America is a global leader in education spending per student. The U.S. dishes out more than $12,500 per student on secondary education -- roughly a third higher than other developed countries. From 1984 to 2014, annual federal outlays for elementary, secondary and vocational education skyrocketed from about $6.5 billion to more than $40 billion -- a 176 percent increase after adjusting for inflation.

So if money’s not the problem, then who’s to blame for America’s failing schools?

Teachers unions. America’s largest teachers unions -- the National Education Association (NEA) and American Federation of Teachers (AFT) -- are notorious for protecting underperforming teachers through generous tenure policies. According to one estimate, only 1 in 500 tenured teachers is ever fired for poor performance. That comes out to 0.2 percent. Lawyers and doctors with advanced degrees often suffer more from underperformance.

In California, roughly 98 percent of teachers attain tenure -- or "permanence" -- after two years of employment. Yet research shows that, from 2004 to 2014, only 19 out of about 300,000 tenured California teachers were dismissed for poor performance. In states like Mississippi, it only takes a teacher one year to secure tenure. This leads to last-in, first-out firing procedures which disproportionately punish the youngest teachers who haven’t been tenured yet -- no matter their qualifications or experience. And it indirectly punishes American students, who lose out on learning from up-and-coming teachers.

But when education proponents propose tenure reform and merit-based pay -- which would reward the best teachers while weeding out the worst -- they are vilified by union bosses. AFT President Randi Weingarten recently claimed that supporters of such policies have “turn[ed] scapegoating into an art form.” "Tenure is not a job for life," Weingarten often argues -- despite the statistics showing otherwise.

She's not alone. NEA President Lily Eskelsen Garcia routinely urges union members to “[stand] up to the scapegoating of educators and [demand] more respect,” while claiming that critics of overly generous tenure policies are liars.

Big Labor has committed to upholding the status quo, bankrolling politicians who oppose school choice and other fixes to our struggling schools. Meanwhile, the children of many dues-paying union families suffer as well. Higher-quality private schools are much less accessible to the family of a unionized store clerk than the local public school.

The NEA and AFT’s Super PACs have spent more than $185 million on political activities and lobbying since 1990 -- almost $23 million of it coming in 2016. This doesn’t even take into account millions more in non-PAC expenditures, which teachers unions use to fund the Democratic Party and an array of closely aligned special-interest groups. In 2015, AFT contributed nearly $1.5 million to the Democratic Governors Association and $400,000 to the Democratic Legislative Campaign Committee, while the NEA dished out more than $725,000 to Catalist -- a leading Democratic data firm.

The NEA ranked as the top organization funding Super PACs during the 2014 election cycle -- well ahead of conservative power players such as the Republican Governors Association (No. 18), Freedom Partners (No. 55), and American Crossroads (No. 66). (The AFT finished a respectable No. 13 on the list.)

And the overwhelming majority of teacher union money going to Super PACs -- 99 percent -- props up the Democratic Party and liberal advocacy groups opposing education reform. The NEA contributed a grand total of $0 to Republicans and conservative causes in 2014. (Yet the most recent NEA survey of public school teachers found that 55 percent of teachers characterize themselves as “conservative” or “tend to be conservative.”)

America's students deserve more than politics as usual.