The start of classes at Penn Manor High School in Millersville was delayed by three hours Thursday while firefighters cleared the building of smoke.
According to the district's website, a portion of the building filled with smoke after a small, localized fire in a clothes dryer at the school.
District spokeswoman Cindy Rhoades said cleaning rags were in the dryer when the drum stopped spinning and the heat stayed on, causing the rags to catch fire.
There was no damage to the building.
All other schools in Penn Manor were operating on a normal schedule.