More than a hundred firefighters were called out just before 4:00 a.m. Thursday to Penn Wood High School's Green Avenue campus in Lansdowne.
They say there was an explosion in one of the classrooms as firefighters were working to put out the blaze.
"As our crews were going the first floor, they heard what appeared to be an explosion, we're not quite sure what term they are using right now," Lansdowne Fire Chief Wayne Worley said.
Some 13 volunteer fire departments responded to the 2-alarm fire before it was finally brought under control. By then, damage to the entrance way of the school was extensive.
Officials say one classroom was destroyed and there was extensive smoke and water damage throughout the building.
"It's devastating. We're not a very rich district; we're probably one of the poorest districts in the state," School Board President Diane Leahan said.
School officials say damage is such that the high school for 11th and 12 graders will remain closed for the rest of the school year and they are working on a plan to hold classes at their Cypress Sreet campus in Yeadon.
Meantime, parents are alarmed not just by the bomb threats, but now arson.
"I'm terrified. I'm scared for my children to go back to school," parent Janell Smart said.
Meanwhile, seniors who were getting ready to graduate & planning for their prom next month were devastated that someone would do this to their school.
"It distracts most of us and upsets us because a lot of us are getting ready for the prom, our trips, maybe our AP tests that are coming up in May," Senior Class President Adrienne Thompson said.
Authorities do have surveillance footage that they are working on enhancing. Meanwhile, the school district has hired around the clock security for its schools in the wake of this arson fire.
The school superintendent is offering $1,000 out of his own pocket for information leading to an arrest.
There is no school for high school students on Monday or Tuesday.