The Montgomery County District Attorney's office said that number could change as the assessment continues.
That initial figure came after an inspection of the building revealed significant structural damage, the DA's office said.
The May 24th fire was contained to the F-Pod area, school district officials said in an earlier statement, but the band storage area and classrooms on the first floor were damaged. A large part of the building was also exposed to smoke.
More than 3,000 students, along with faculty and staff, were evacuated. There were no injuries.
The 17-year-old accused of starting the fire is in the county juvenile detention center. He faces an adjudication hearing on Friday.
The boy could be incarcerated or supervised by the court until he's 21 if he's found to be delinquent, the juvenile court equivalent of a conviction.