Mexican students studying to be teachers released a hostage on Wednesday—in the municipality of Nahuatzen—due to concerns about his health. But they continue to hold five others. The students are supported by the Michoacán State Teachers Organization, which warned that the remaining captives, who are state policemen, would be freed only when a demand for 1,200 new teaching jobs is met.

WSJ's "Americas" columnist Mary Anastasia O'Grady talks with James Freeman about the tension between teachers unions in Mexico and newly elected President Enrique PenÞa Nieto. Photo: Associated Press.

The Mexican standoff, now a week old, is only the latest example of a teacher-union rebellion against recent amendments to the Mexican constitution aimed at improving public education.

Institutional Revolutionary Party President Enrique Peña Nieto has made it a priority to fix the broken public-education system. But eager reformers are often tested by politically powerful interests in their first year in office. The teachers believe they can make him back down.

Over the 71 consecutive years that the PRI ruled Mexico until 2000, it earned a reputation for heavy-handedness bordering on repression. Now that it is finally back in power, there is pressure on Mr. Peña Nieto to show that his party is kinder and gentler. This may tempt him to tolerate union violence. But the recent constitutional amendments increase transparency and accountability and depoliticize education. This is the change the PRI needs to show the public it supports.

It's easy to see why teachers are up in arms over the amendments. For the first time ever they will be vetted in a comprehensive process that includes proficiency exams. Lifetime tenure will no longer be guaranteed from the day a teacher graduates from a teaching college. Teachers will not be allowed to pass their tenured posts to relatives, the prevalent practice of selling a teaching post will be outlawed, and promotions will require evaluation. In short, teaching is to be like a real job, where performance matters.

Accountability is a foreign concept for many who go into teaching, which explains why teaching students are part of the rebellion. In April, violence broke out in Chilpancingo, the state capital of Guerrero, when the state legislature refused a request by activists to reject the new evaluation process. Union thugs vandalized property. They also blocked the highway that runs from Mexico City, through Chilpancingo, to Acapulco, with serious economic consequences. Most teachers unions at least pretend to care about their students. Many striking teachers in Mexico just walked off the job, leaving children and parents in the lurch.

Teachers block a major highway in Chilpancingo, Mexico, in April.

Mr. Peña Nieto is leading a nation that long ago outgrew the labor laws that govern teaching. When the North American Free Trade Agreement was born in 1994, many expected Mexico to use it to become a magnet for low-skill, low-tech industries. But openness raises living standards, and destinations outside of Nafta soon became more attractive for investors seeking low-wage labor.

Mexico naturally moved up the food chain. Today it increasingly hosts middle- and high-tech industries, including aerospace. According to the ministry of the economy, "There are over 190 companies within the aerospace industry operating in Mexico, employing nearly 30,000 workers" and the aerospace market is undergoing "rapid growth."

Will Mexico have the human capital needed for that growth and the other economic changes that will accompany it? The education ministry claims that every year 90,000 Mexicans complete graduate programs in engineering and technology. But the public-education system could hold the country back.

In very poor countries, access to education is the first hurdle to clear. But step two, which Mexico now faces, is the quality challenge. Claudio X. González, president of Mexicanos Primero (Mexicans First), a nongovernmental organization, is working to build public support for reform. He says "only one quarter of each generation finishes high school," and only "10% finish their college degree." What is more, "up to 80% of each generation fails or barely passes international tests in reading comprehension, math and science."

Mexico ranks 34th out of the 34 countries in the Organization for Economic Cooperation and Development in basic education achievement. Mexicanos Primero reports that Guerrero, Oaxaca and Michoácan, where much of the union violence has taken place, have the nation's worst secondary-education outcomes.

Notes Mr. González: "No country in the OECD spends as much on education, as a percentage of GDP, as Mexico. Still, we have no reliable registry of teachers and close to 160,000 people [are paid] a salary as teachers but never step into a classroom."

Mexico's elected representatives have voted to reform a corporatist education model built in the 1930s. Now the state is charged with enforcing the change. It should also prosecute vandals and kidnappers. If it does, it won't only defend the interest of millions of children but it will also be a step closer to implementing the rule of law in a country where it is sorely lacking.