The Fire and Emergency Service says gas was being released from LPG bottles inside a 20-foot container on the school's grounds.
A spokesperson says for some reason, someone decided to vent the cylinders and drain the gas out of them - an unwise thing to do.
The college and several neighbouring properties were evacuated at 2.30pm as firefighters worked to contain the leak.
Firefighters say the site was safe just after three o'clock.