Strabane Grammar School is facing a repair bill running into tens of thousands of pounds after vandals broke in on 12 July.
Part of the building was set on fire and 70 windows were smashed, in what was the latest in a series of attacks.
The lower floor of the school's gate-house on the Liskey Road was "gutted", according to staff.
It is believed to be the fourth attack on the school this month.
Vice-principal Richard White said he was shocked, but determined to have the school ready for pupils returning in September.
"There was a lot of paint damage where they had fired paint bombs at the school," Mr White said.
"We saw that they had gained access to the assembly hall and the main building.
"They damaged extensively the trophy cabinet... and there's been some crystal broken."
The Chair of Strabane District Council Brian McMahon said the cost of repairing the damage will "place a burden" on the school's budget.
"They will also have to increase security and that is going to be another extra cost," he added.
Mr McMahon said he "could not rule out the possibility" that the attack had a sectarian motive.
Sinn Fein MLA Michaela Boyle said she believed the damage had been caused by youths bent on recreational violence.
"It's just deplorable. It's sickening," she added