Dozens of students were arrested in the capital and protest leaders said half a million students occupied hundreds of schools all over Chile calling for free bus fare, free college entrance exams, more teachers and improved school buildings.
"We are protesting on behalf of our school. The bathrooms are disgusting, you can't even take a shower in the locker room, and they don't do anything about it," said Bernardo Ferrada, 15, his nose and eyes burning from tear gas.
Ferrada said he and 25 other students from the Arturo Prats High School in the middle class neighbourhood of Puente Alto joined a march headed to the national palace before police sprayed them with tear gas from armoured vehicles.
Protests began two weeks ago when students took over a few schools in the capital, sleeping overnight in classrooms and eating food brought in by sympathetic parents.
The movement spread all over the country and has turned into the biggest street protest faced by new President Michelle Bachelet.
Local media said Chile had not seen student protests on this scale since the early 1970s when Socialist President Salvador Allende was in office.