ELEVEN students and a teacher are being monitored in hospital after an explosion at a high school science lab in the NSW southern highlands.

Paramedics were called to the Moss Vale school at 12.43pm (AEST) on Tuesday after a glass flask containing the chemical sodium hydroxide exploded.

The science class of 23 year 7 students was doing a routine “fountain” experiment when the accident occurred, Tom Cooper of NSW Fire and Rescue said.

“They were conducting a fountain experiment where you have a flask down below and some tubing that goes up into another flask above it and you try to create a gas,” he told Fairfax radio.

“The gas inside the top of the flask has caused it to explode and basically the students have been splashed with one of the chemicals.”

The students suffered slight burns to their hands and faces, he said.

Emergency crew members wet the students down and they were transferred to Bowral hospital where they are in a stable condition.

The families of the students have been informed and officers from Fire and Rescue NSW and Goulburn Police were investigating.