Fire and Rescue NSW and Hazmat attended the scene at St Edwards College at East Gosford after a gas leak earlier this morning.
Emergency services were called to the school at approximately 9.30 am and quickly located a leaking LPG Cylinder.
Approximately 60-80 students had to be evacuated while the LPG cylinder was removed from a science lab and the classroom ventilated.
Below the area with the leaking Cylinder, an unidentified white power was spilled on the floor and Hazmat were called to investigate. Some staff complained of nausea and three patients were treated on the scene by paramedics.