Police said a male juvenile, possibly a student at the school, broke into Claremont High around 6 a.m., and set a chemical fire in a physiology, science lab.
"There was something in the room that said flammable. They dumped it on some papers on the ground and lit that off," said San Diego Battalion Chief, Matt Nilsen.
The fire department quickly extinguished the blaze, and contained the damage to just one classroom. Authorities said the suspect appeared to have gone on a rampage in other classrooms, as well. While firefighters put out the flames, police arrested a 16-year-old they found running inside the school. He is in custody and being questioned.
The school informed parents around 7 a.m., Wednesday that classes would not be canceled, and asked students to gather at the school's football field until an all-clear was given by the fire department. About 30 minutes later, the school informed parents and students that classes had been canceled for the day.
The investigation remains active. Classes are set to resume Thursday.