The fire happened just before 8:00 p.m. Monday night. The fire marshal says a wooden window frame was set on fire from the outside.
Firefighters had to break into the school to fight the flames. Photos of the scene sent to 6abc.com showed damage to the school's auditorium.
Classes were cancelled on Tuesday and the auditorium was sealed off as workers ventilated the building and made temporary repairs.
Repair and cleanup crews have been on the job since 8:30 Monday night.
The timing couldn't have been worse. 9th, 10th and 11th graders are scheduled for their PSAT tests on Wednesday.
However, officials say the doors will be open on Wednesday, just in time for the test.
Meanwhile, on Tuesday, students were at a loss to explain why anyone would do this.
"Why? It's unnecessary. You're trying to burn down a place trying to educate your children for the future? It's foolish," said 12th grader Anna Thum.
Another student couldn't imagine someone risking lives that way.
"What if somebody was there on behalf of our school and they were still in the building? What if a janitor was in there?" said 11th grader Hyeju Lee.
Unfortunately, the students here will now have to make up for lost time.