Work stoppage in some 300 secondary schools in Madrid is to last at least two days and perhaps three, and teachers elsewhere in the country also plan strikes or protests this month against budget cuts.
Teachers say education should be spared as Spain tightens its belt to resurrect its economy, allay fears it might need an international bailout and reinvent itself for the future with a modern, educated workforce after the collapse of an economy fueled largely by a real estate bubble.
"We are on strike to improve state education. It is not true that we are on strike because we have to work more.
The timetable is the same as we had last year. What we want is better conditions for public teaching," Pilar Hortal, a 57-year-old English teacher standing at a picket line in Madrid, told The Associated Press.
The teachers' branch of the UGT union said most Madrid teachers were honoring the strike call.
Education in Spain is largely run by regional governments, many of which are debt-laden. The one in Madrid hopes to save €80 million ($110 million) with staffing cuts. It and the others making budget cuts are mostly run by the conservative Popular Party.
The central government of Socialist Prime Minister Jose Luis Rodriguez Zapatero, which has enacted austerity measures of its own, opposes education cuts.
The strike's immediate trigger was an order from the Madrid regional government forcing teachers to give two extra hours of class work per week, going from 18 up to 20.
Their actual work week remains unchanged at 37.5 hours.
Unions say the extra classroom hours mean several thousand backup or temporary teachers will not be hired this year.
Teachers will have less time to prepare classes or meet with students and parents, and can't use auxiliary colleagues to break big classes up into smaller groups for trickier subjects like science or foreign languages.
Unions say some teachers are being assigned to teach subjects they know nothing about.
Spain, meanwhile, easily raised €4.45 billion ($6 billion) in an auction of short-term debt, although higher borrowing rates reflected investor worries over the impact of Europe's debt crisis.
The Treasury had wanted to sell between €3.5 billion and €4.5 billion in the auction.
It sold €3.59 billion in 12-month bills at an average interest rate of 3.59 percent, up from 3.34 percent at the Aug. 16 auction. Demand outstripped the amount actually sold by a ratio of 2.8.
Spain also sold €870 million ($1.2 million) in 18-month bills at an average yield of 3.8 percent, compared with 3.59 percent on Aug. 16. The oversubscription rate was 2.7.