The Mexican government and the powerful SNTE union signed Tuesday an accord obliging more that 1.2 million teachers to be evaluated every three years.

At a ceremony in the capital's National Museum of Anthropology, President Felipe Calderon said that this is a "historic agreement to aid the change in education the country needs."

Evaluation will be applied in stages to teachers in both public and private schools at all levels from pre-school to high school, the president said in congratulating the teaching profession for taking on "the challenge of quality education."

The evaluation "will allow us to better know the strengths and detect the weaknesses" of the educational system, in order to "get on the right road" to finding out what we have to modify, improve or change in order to make progress with the educational transformation of Mexico, he said.

The goal is to improve students' education through better teacher training, Calderon said, recalling that up to now only three out of every five teachers has ever undergone a process of evaluation.

The results of the evaluation, which will be published, will also be taken into account to reward teachers with incentive pay.

Attending the ceremony were Education Secretary Alonso Lujambio and SNTE chief Elba Esther Gordillo.

In the 2009 test of the Programme for International Student Assessment, given every three years to 15-year-olds in 65 countries, Mexico ranked 48th.