Mexican President Enrique Pena Nieto has signed into law the biggest overhaul of his country's education system in 50 years.
The 1.5 million-strong education union SNTE said the law, passed on Monday, was an attack on job security.
The new system aims to create uniform standards for teacher hiring, boost the number of students in higher stages of education and create a national database of the number of schools.
Under the old law SNTE general secretary Elba Esther Gordillo hired and fired teachers.
But the union's opponents allege that there are thousands of phantom teachers on the payroll to inflate teachers' wages.
Ms Gordillo did not offer any immediate comment on the new law, but has long pledged to oppose any threat to her members from the reforms.
She has also said that Mr Nieto has been seeking to discredit publicly run schools in favour of private ones, which are more lucrative for big business.
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