IN 1918 Mexico became one of the first countries in the world to declare an annual Teachers’ Day holiday. The 100th anniversary, on May 15th, was a politicised affair. Enrique Peña Nieto, the lame-duck president, gave a speech at an official celebration. Meanwhile, members of the National Co-ordinator of Education Workers (CNTE), a strident teachers’ union, took to the streets. They were protesting against the education reform Mr Peña signed in 2013—whose effects are now being felt both by the students he hoped to aid and by the unions he tried to weaken.

Campaigning for Mexico’s presidential election, due on July 1st, has concentrated on crime and corruption. But the biggest issue on the ballot is arguably the fate of the structural reforms that Mr Peña and his legislative allies passed in 11 policy areas. The president’s aides tout his education law as the most popular of them all.

Yet it could well be the first to go. Even though the national teachers’ union, the SNTE, formally backs the reform, the CNTE, a militant branch strong in four poor, southern states, is intent on getting it reversed. Its members are planning an “indefinite” strike for June 4th, which would affect 13,000 schools. The final presidential debate, on June 12th, will focus on education. Andrés Manuel López Obrador, a left-wing populist who is the heavy favourite to win, is sure to reiterate his oft-expressed wish to do away with the law.

Mexico may venerate its teachers, but that seems not to have translated into learning. By global standards the country’s education system is an underperformer. Its 15-year-olds’ scores on the PISA test, which measures proficiency in science, reading and maths, lag far behind those of European countries like Bulgaria and Romania, which spend similar amounts per student on education. Even within Latin America, its scores are only slightly better than those of Colombia, whose per-student budget is 45% lower.

Learning the hard way

Given the SNTE’s historical stranglehold on Mexican schools, it is surprising that the country does not fare even worse. Before 2013 the union held a majority of seats on “hiring commissions”, enabling it to control who entered the profession. Teachers faced no performance reviews and could not be fired. They could transfer their position to anyone they chose: classified ads in newspapers often offered teaching jobs for sale. The government did not even have data on how many instructors it paid. In 2014 a census of teachers found that 300,000 recipients—whose estimated salaries made up 10% of the federal education budget—had left, died or never existed.

For decades the SNTE used its clout to preserve this system. Its 1.5m members tended to vote in unison, and persuaded parents to do the same. One study found that candidates the SNTE backed received a 2% boost in polling stations that were schools. The SNTE’s support for Mr López Obrador’s victorious rival in the presidential race of 2006 may have been decisive.

Mr Peña, however, has turned the tables. In 2013 prosecutors arrested Elba Esther Gordillo, the head of the SNTE, on embezzlement charges. Although she denies wrongdoing and is awaiting trial under house arrest, the show of force may have encouraged the SNTE to back his reform.

The law instructed the federal government to pay teachers directly, which stopped unions from handling their pay cheques. It also required candidates for jobs and promotions to undergo a written test about teaching methods and academic subject matter. Some states, like Puebla, now hold public events to announce which candidates scored the best. It has already promoted 2,000 people to the role of principal on this basis. Mr Peña wants to launch a similar platform nationwide.

It is too early to evaluate the effectiveness of the reform. The next PISA results are not due until 2019. And even if the policies are working, test scores may not improve. It will be a decade before students taking the exam will have been taught mainly by instructors hired on merit.

At this stage, the only credible benchmark is the rate at which the reforms are being implemented. It has been relatively slow. In part, that is because of the compromises Mr Peña made to win the SNTE’s support. For example, teachers hired before 2013 still enjoy lifetime employment, though they can be moved to administrative roles. No one has yet been fired for failing an evaluation, or for skipping work three days in a row. Phantom teachers remain on the payroll, in smaller numbers. Only a third of new teachers participate in a mentoring scheme.

The reforms have also been stymied by fierce opposition. CNTE members are boycotting their evaluations and refusing to administer tests to their students—sometimes under threat from menacing union thugs. Strikes have interrupted every school year in Oaxaca at least once since 1994; the cohort entering in 2012 lost nearly a year’s worth of classes over six years of primary school. Despite rulings by the Supreme Court that a teacher’s right to strike does not trump a child’s right to learn, officials have not dared to fire them. They have reason to fear the consequences: in 2016 eight CNTE protesters died when police tried to clear a road they were blocking.

The greatest threat to the reforms, however, comes from Mr López Obrador. Leading the polls by up to 25 percentage points, on May 13th he vowed that “the so-called educational reform will be cancelled”. He also demanded justice for “political prisoners”—as union officials locked up for corruption call themselves. He argues that the law is a gateway to school privatisation, and that poor states like Oaxaca should not be held to nationwide standards.

The reformists’ best cause for hope is the split between the unions. The SNTE, the far bigger of the two, has cast its lot with the new policies, and may be loth to change course. The best-case scenario for supporters of the law is that Mr López Obrador is only promising repeal to shore up votes, and will merely tinker with the reform once in office. That would unleash a new wave of fury from the CNTE. But it might also give the law enough time to work, and become politically sacrosanct by the presidential election of 2024.