SANTIAGO, Chile (AP) — A homemade firebomb exploded inside a public high school in Chile on Wednesday, igniting a blaze that injured at least 35 students, several critically, firefighters said.

A group of students at the school in central Santiago, the capital, were making Molotov cocktails in the bathroom to be thrown at a protest later when one exploded, said police official Lt. Col. Fernando Albornoz. It was not clear what caused the blast.

Capt. José Manuel Estefane with the fire department said 11 students at the prestigious Barros Arana National Boarding School were hospitalized in serious condition, with burns affecting their respiratory systems.

The rest of the students had lighter burns, authorities added, while a teacher trying to help the students collapsed. Firefighters quickly contained the blaze and blocked roads around the scene.