Around 28 students and teachers were admitted to hospitals in Thrippunithura and Ernakulam, police said.
School authorities said some foul smell started emanating in the school premises in the morning and it got worse at around 9.30 AM.
They told TV channels that they soon alerted police and health authorities. Some of the students swooned, they said.
Police said the state pollution control board has initiated a probe as the incident occurred in the area where many companies, including the Bharat Petroleum Corporation Ltd (BPCL) and fertiliser PSU FACT, are located.
District authorities have declared a two-day holiday for the school.