Eight mobile classrooms were destroyed at the former St Patrick's Academy building in Dungannon last night.
More than 40 firefighters managed to prevent the blaze spreading to the main school building.
Sinn Féin MP Michelle Gildernew urged local people to exercise caution while the fire was being tackled at Killymeal Road shortly after 8pm.
The Fermanagh South Tyrone said, “Whether accidental or deliberate, fires are incredibly dangerous and destructive so I would urge everyone to exercise extreme caution.
“I would also like to appeal to everyone to report any suspicious activity as the deliberate lighting of fires is reckless and completely unacceptable.
She added, “I would like to recognise the selfless efforts of the emergency services who are at the scene tackling the fire.”
The cause of the fire is still being investigated although there has been speculation that it may have been deliberate.