Officials at Pennridge High School say the first student became sick around 10 a.m.
School nurses then treated three other students with similar symptoms.
Officials say the school building and the district office next door were evacuated as a precaution.
Firefighters responded a short time later.
They gave the "all clear" after testing the building for hazardous materials.
Classes resumed around 11 a.m.
It's not yet clear what caused the students to fall ill.