At Madrid's state-funded Jose Luis Sampedro high school, teachers proudly recall pupils who have gone on to success: the maths prodigy, the virtuoso pianist, the NASA employee.

It is a record, they fear, that lies in peril because of regional government spending cuts.

Ahead of Sunday's general election, teachers across the Madrid region are taking to the streets in a "Green Tide", a reference to their green T-shirts that declare: "Public Education of all, for all".

Class sizes at the Sampedro high school have swelled to up to 38 pupils, and 13 teaching posts have been cut under measures introduced by the regional government, led by the conservative Popular Party.

Madrid teachers have been ordered to give 20 hours of lessons per week, up from 18 previously, to save money on supply staff.

"They are squeezing tightly in every way possible to cut as much as possible," said Sampedro maths teacher Inigo Echenique, 54.

"What angers us the most is that their aim is to limit the role of public education in the Madrid region," he said, accusing the regional government of favouring private education.

The central Socialist government has ordered Spain's 17 regions to cut public deficits to 1.3 percent of their economic output.

In many regions, the health sector has been hit hard.

But in Madrid, the biggest flashpoint is schools.

With market pressure rising on Spain to strengthen its public finances, the cuts are likely to go even deeper after Sunday's general election, which opinion polls say the Popular Party will win comfortably.

The Socialist government cut public sector salaries by five percent in May 2010.

Now the rival Popular Party promises in its manifesto "an austerity plan involving the whole public administration, to eliminate excess spending and provide incentives for efficiency."

The party has given few details, although its leader Mariano Rajoy has said health and education will be spared.

Fernando Vallespin, a political scientist at Madrid's UAM university, said Spain had an unusually large private education sector.

At the time of General Francisco Franco's death in 1975, "practically the whole education system was in the hands of the church", and since then state investment in education has been slow to catch up, he said.

The labour ministry says one in two pupils in Madrid goes to a private school and three out of 10 in Spain overall.

But according to the Organisation for Economic Cooperation and Development, Spanish pupils' exam results rank below average in the rich world.

"In a country like this, with unemployment at more than 21 percent, it is a social problem," says another Sampedro school teacher, technology master Jose Sebastian Trocoli.

The teachers have called a demonstration Thursday and a strike for November 23. They say the movement will grow if the Popular Party takes power and tightens spending further.

"This protest is going to continue one way or another. Maybe in the education sector in Madrid, or maybe the spark will light up another part of the Spanish state or another sector," said Echenique.

"But as long as public services are under attack like this, it will provoke the same response."