According to a school spokesman, the emergency services were called to the Shooting Field campus after a 'contained chemical spill in a technology room' this morning (January 17).
The spokesman said: "SGS staff evacuated the room and surrounding areas immediately and then, under the advice of the fire service, cordoned off the block as a precautionary measure. Following a thorough assessment of the area, the school was deemed safe to remain open by the fire service.
"All students on site are safe and all lessons were re-timetabled for the rest of the day.
"The safety of our students and staff remains our utmost priority."
A spokesman for West Sussex Fire and Rescue Service confirmed crews had been called to a minor chemical incident at 11.58am.
The chemical has been dealt with, the spokesman said, but crews remained at a cordoned off area of the scene so classrooms can be ventilated.
Steyning Grammar School recently announced it was selling its Church Street site. Read more here: Steyning Grammar School to sell historic Church Street site as it looks join academy trust