The boy who gave the tip was one who was trying to be recruited,
" Castor added. "He was a friend of the boy in the loosest terms."
Officials said they think the tip was prompted by Wednesday's shooting at a school in Cleveland, Ohio.
"We have no information at this point that leads us to think this is other than an isolated individual who was trying to recruit others to help," Castor said.
School officials said the boy is not a student at the school.
"This was a youth in the community who has not been enrolled in school since spring of 2006," according to Dave Sherman, spokesman for Plymouth Whitemarsh High School.
The school has an enrollment of 1,591 students in grades nine through 12. No classes are being canceled because of the incident, school officials said Thursday.
Plymouth Township is about 15 miles northwest of Philadelphia.