Police said Friday that the weapons were discovered Wednesday at William Penn High School.
Investigators say school administrators removed two students from their classrooms after seeing an Instagram photo showing the students posing with a handgun in a school bathroom.
School staff searched both students' bags. They found a 16-year-old boy had a realistic looking gas-powered pellet gun. A 15-year-old boy had an unloaded 9mm handgun. After further investigation, they determined a third student had originally brought the handgun to school, and school staff found ammunition in his book bag.
All three students are being charged with possession of a weapon in a school zone and related charges.