The fire seriously damaged the interior main entrance, the flames so hot they melted metal closures on the front entrance doors. Two offices and a classroom also suffered considerable fire, smoke, and water damage, officials said.
Located on Green Avenue, the school also serves the other communities in the William Penn School District - Aldan, Colwyn, Darby, East Lansdowne, and Yeadon.
The yellow-brick, two-story structure, home to the district's 750 juniors and seniors, had been the target of recent bomb threats, Kortan said. A separate campus on Cypress Street in Yeadon houses freshmen and sophomores.
Kortan said notes had been taped to the entrance warning that no one should enter, and phone threats were traced to cellphones that were out of service but still capable of making emergency calls.
School officials hired off-duty police officers to patrol the building at night, but the threats ended two weeks ago and the patrols were suspended.
"Today there was no threat, simply a fire," Kortan said.
Noting that a back door had been punched in overnight and that fire burned in multiple locations, Kortan declared the blaze suspicious. Local, county, and state fire marshals were called to investigate.
The school district, he said, has "a very good camera system," adding that surveillance video could prove crucial to the investigation.
Joseph Bruni, district superintendent, said the tentative plan calls for classes on a split schedule, starting next week, at the Cypress Street location. Juniors and seniors would attend in the morning, and ninth and 10th graders in the afternoon.
The prom, along with the graduation ceremonies normally held at Temple University, will continue, but plans might require some adjustments, Bruni said.
The damage to what he called "the center of the community" has been devastating. Other organizations that use the school's facilities, such as the Boys and Girls Club, will be displaced until repairs can be made. He expects that students will not return to the school until September.
The building dates to 1927 and was built by John McShain, a Philadelphia native whose work also includes the Pentagon, the Jefferson Memorial, and the John F. Kennedy Center for the Performing Arts.
The building had undergone an extensive renovation in 1982, adding new carpets and windows but not a sprinkler system.
Henry said he first heard about the fire from a friend who called him at home with the news. He said he had no idea who would set a fire at the school but speculated it was someone with a vendetta, perhaps a student who had been expelled.
The Lansdowne resident, who is headed to the University of Arizona to study business, had nothing but praise for his school, saying, "the staff and teachers are great." It was the kind of school where you could have fun, he said.
"To end my senior year like this -," he said, unable to finish the sentence.
Contact Mari A. Schaefer at 610-313-8111, firstname.lastname@example.org, or @MariSchaefer on Twitter.