Police said that a 1.5 litre bottle of nitric acid broke when it fell to the floor of the 10th grade Chemistry class before lunchtime on Monday, adding the circumstances of the accident remain unclear.
About 1,100 students were evacuated from the school in the German state of Lower Saxony. Fire department experts have begun clean up efforts.
Thirteen students and one teacher were treated at a hospital for possible allergic reactions to the nitric acid, which is a corrosive chemical that can cause severe burns to skin, eyes and mucous membranes. Breathing nitric acid can also burn the lungs.