The schools’ decline has been years in the making, despite the pledge of Mexico’s 1910 Revolution to spread literacy. For many, the blame lies with the National Education Workers Union, whose members are valued for political fealty, not pedagogic skills. Formed in 1949, the union became one of the pillars girding the Institutional Revolutionary Party, or PRI in its Spanish initials, the party that ruled Mexico for most of the 20th century and whose candidate is thought all but certain to win the presidency again in voting July 1.

Even with the PRI out of power for the past 12 years, union clout didn’t diminish. By law, all teachers are enrolled in the union ranks, and 1 percent of their salaries go to union coffers.

“It might have a bigger budget than many states. In fact, it is a state within a state,” said David Calderon, the general director of Mexicanos Primero, a civic group that’s pushing for deep educational reforms.

His group estimates the revenue flow from dues, government contributions and business investments to the union at 1.8 billion pesos each quarter, or about $130 million. Other experts offer lower estimates. Since union leaders aren’t required to detail how they spend the money, no one outside the union knows for sure.

The union holds no truly democratic elections, and Calderon described it as “a mix of the Soviet model and Chicago in the 1920s.”

“Why do we have a union like this? Because it is very convenient for our presidents,” he said. “It is an electoral force and also a mechanism of control and mediation with state governments.”

Since reforms in the 1990s that devolved power to states, presidents have faced uneasy confrontations with elected state leaders, and “weak presidents need a strong ally to control the governors,” Calderon said.

The power of Mexico’s teachers union has few parallels in the world, and its chief, La Maestra – “The Teacher” – as Elba Esther Gordillo is known, wields influence far beyond the sphere of education. Indeed, her union began its own political party in 2005, the National Alliance, and it now has five deputies and four senators in Congress. Its candidate in the July 1 elections polls in fourth place.

The union wields its muscle to protect teachers, not to improve education. By some estimates, it has taken 21,000 teachers out of the classroom to handle union business, organize political rallies, chauffeur union leaders and broker the buying and selling of teaching jobs.

Teachers are more beholden to union delegates than they are to principals, and some routinely skip classes, leaving pupils on the playground all day.

“Teacher absenteeism is a real problem,” said Francisco Bravo, an elementary school principal in Mexico City who leads a branch of a dissident group. Union leaders “take teachers out of the classroom to help campaign.”

A third of elementary school teachers say their fellow teachers “occasionally or frequently have unjustified absences” or show up late. Fully two-thirds of high school teachers say that’s the case.

Frustrations run high at many schools, as administrators scrape for money and parents bristle at unforeseen fees.

In San Blas, a coastal city in Nayarit state, Principal Fernando Flores takes a visitor through his primary school, showing a classroom jammed with 43 students and pointing to air conditioners, overhead fans and inkjet printers – all broken.