The teachers, along with a number of students had all reported feeling nauseous following an early morning emergency evacuation of the school.
According to Headmaster, Neil Morton, the gas leak was caused by a malfunction in one of the school's central heating boilers.
A letter, explaining in detail what occurred on Friday was sent out to parents and guardians later that day.
In his letter, Mr Morton explained that at around 8.40am there was a pervasive smell around the back classroom block of the school.
At 8.45am instructions were issued for an emergency evacuation.
Windows were opened to ensure ventilation of the building.
All students were directed to meet in the parade ground where they remained for around an hour, and were supervised, until given the all clear.
Three fire appliances, two from Enniskillen Fire Station and one from Irvinestown were deployed.
A gas monitor was brought into the school to ensure that all races of carbon monoxide were removed from the premises.
Normal school convened at around 9.55am. However three teachers who had been in the staff room above the boiler room reported feeling unwell after 9am and were sent to the South West Acute Hospital for check-ups.
They returned after 10am after being assured they had not been exposed to poisonous gases.
According to Mr Morton, a number of students later reported feeling unwell. Their parents were informed.
"At all time, the safety of all of the community was foremost in our considerations," Mr Morton wrote in his letter.
The defective boiler has since been repaired.