Four pupils and a teacher have been taken to hospital after an explosion in a classroom.
Emergency services were called to Heolddu Comprehensive in Bargoed shortly before 0900 GMT after reports of an explosion.
The pupils and their teacher were "washed over" by an ambulance crew because of the chemicals involved.
A spokeswoman for Caerphilly Council said they were only taken to hospital as a precaution.
The teacher was performing an experiment in the laboratory's fume cupboard and the pupils were observing when the incident occurred, the spokeswoman said.
She added: "The incident resulted in a small amount of chemicals being splashed onto the teacher and pupils' clothing.
"As a precautionary measure, the school - following its procedures for such an incident - sent the five individuals to hospital and called the fire service to cleanse the area."
The pupils and teacher were taken to Prince Charles Hospital in Merthyr Tydfil.
South Wales Fire and Rescue Service sent appliances from four different stations and a chemical incident unit.
Firefighters ventilated the chemistry laboratory to make it safe.
A Welsh Ambulance Service spokeswoman said it had been a "small explosion".
She added: "At the scene we checked those involved for breathing difficulties and gave them a wash over because of the chemicals involved.
"We took a number of patients to casualty but only as a precaution. As far as I know, nobody was admitted to the hospital."
Deputy Head teacher John Woodier said: "This incident is unfortunate, but the procedures for such an occasion are in place.
"We are satisfied that everything has been done to mitigate any further potential injury to staff or pupils.
"We are grateful for the swift response by the fire and ambulance services."