A FEW weeks ago, flyers appeared around Denver in anticipation of a school-board election that was held on November 7th. At first glance, they appeared typical, bearing the smiling face of the candidate next to a short explanation of why she deserved to win. “Jennifer Bacon is the only candidate who has actually taught in public schools,” it read. But above that ordinary proclamation was a more surprising claim. Next to an image of an open safe containing stacks of $100 bills the flyer blared: “Rachel Espiritu’s campaign is funded by dark money from groups outside Colorado tied to Donald Trump and Betsy DeVos.”

Elections to choose school boards, which shape education policy at the local level, have historically been sleepy, low-turnout affairs. But in recent years they have become contentious, serving as proxies for the rancorous debate between advocates of education reform and teachers’ unions. The reformers champion increasing access to charter schools and expanding educational options in general; the unions oppose such an agenda on the grounds that it could attract students away from districts that bargain with teachers collectively. (Charter schools receive public money but are run independently, usually by non-profit organisations but sometimes by private companies.) In the past few years outside donors with ideological or financial interests in such fights have become involved, focusing attention and money on previously ignored local races.

Chalkbeat, an education news organisation, reported that political committees on both sides of the dispute channelled at least $1.65m into the school-board races that took place on November 7th in Denver, nearby Aurora and Douglas County. Other areas have seen even more expensive contests. In Los Angeles, where three board seats came up for election earlier this year, outside groups poured nearly $15m into canvassing and advertisements on behalf of the candidates. Much of the money came from California Charter Schools Association, which supports charter schools and received nearly $7m from Reed Hastings, the co-founder of Netflix, in the run-up to the election, and United Teachers Los Angeles, a union which opposes charters. According to Carol Burris, the executive director of the Network for Public Education, an advocacy organisation, outside money has also fuelled school-board fights in Louisiana, Minneapolis, and Perth Amboy, a town of just 52,500 in New Jersey.

It is not just the volume of cash being poured into school-board elections that is striking. So is where it comes from. As with political contributions in general, the origins of donations in school-board races are being obscured. The elections in Colorado illustrate how. Political action committees (PACs), which pool contributions from members and put them towards campaigning for or against candidates, are required to disclose their donor rolls. But social-welfare organisations, also referred to as 501(c)4s after the section of the tax code that describes them, are not. Those who wish to fund local races anonymously can direct their money to amenable 501(c)4s, which in turn donate to the PACs. In Colorado, for instance, a PAC called Raising Colorado, which supports the campaigns of charter-school champions, has received donations from only one source: Education Reform Now Advocacy (ERNA), the 501(c)4 arm of a non-profit organisation with its headquarters in New York City and Washington, DC. Who has donated to ERNA is a mystery.

Pedro Noguera of the Graduate School of Education and Information Studies at the University of California, Los Angeles, attributes the growing focus on school-board races to the perception—especially among the very rich—that America’s school system is a failure. American students rank significantly below their peers from other developed countries in science, mathematics and reading. Federal testing data suggest maths proficiency among fourth- and eighth-graders is dipping after years of improvement. Such discouraging numbers, coupled with fears about how artificial intelligence and other technological advances will up-end traditional methods of education, have motivated some wealthy businesspeople to throw money at educational reform.

Sometimes they have tried to do this directly. In 2010, Mark Zuckerberg gave $100m to Newark’s public school system. Much of that was used to launch new charter schools. Last year Mr Hastings established a $100m educational foundation. Others have funnelled money into the hands of organisations and politicians who share their views. In addition to Mr Hastings, Michael Bloomberg, a billionaire and former mayor of New York City, Arthur Rock, a Silicon Valley investor, and heirs to the Walmart fortune have all dedicated large sums of money to campaigning for charter schools.

The backers of these schools insist that independence produces superior academic results. Critics argue that where charter schools do better, that is because they do not enroll the same types of students, such as those who are learning English as a second language or who have disabilities, and that they shed their most difficult students back into the public system. Studies of charter schools’ performance by the Centre for Research on Education Outcomes (CREDO) at Stanford University have turned up mixed results. A report published in 2013 suggested that pupils at charters progressed by the equivalent of only eight additional days of learning per year compared with their peers who are educated at conventional public schools. But follow-up research suggests that charter schools have been much more successful when educating poor children in cities.

Further complicating matters, a recent study in California found that charter schools with unionised teachers may do best of all, a result that is anathema to both sides. That the data are ambiguous means that charter schools’ advocates and unions will continue to scuffle for the foreseeable future. School-board elections are therefore likely to remain fiercely contested. But the intense interest of outsiders is unlikely to boost that of locals. A mere 8.5% of Los Angeles voters bothered casting ballots in this year’s race, the costliest for a school board in American history.