Rocks fly in Mexico over a recently reformed education system.
The reform seeks competency exams, merit-based promotions and seeks to tame a powerful teachers' union blamed by many for hurting Latin America's second-strongest economy.
Teachers fear they could lose their jobs if they don't pass periodical, mandatory tests.
Parents -- some stepping in to replace striking teachers -- stood atop a school rooftop to try to defend it from being overtaken by protesting teachers.
Despite injuries sustained by protesters, police said they were able to maintain order.
(SOUNDBITE) (Spanish) OAXACA MINISTER OF PUBLIC SAFETY, MARCO TULIO LOPEZ, SAYING:
"The police at all times tried to keep the order inviting both parties in conflict to calm their violent attitude. I personally was here on this corner talking to people but there was not reason or capacity for dialogue."
Mexico's record on student achievement is one of the worst among countries by comparison
