Rescuers were called to the Pottsgrove Middle School at 10:18 a.m.
Superintendent Shellie Feola said in a statement that it all began when three students participating in choral practice became light-headed and fainted.
Emergency responders were contacted as additional students reported lightheadedness.
A total of 22 students were eventually transported to two area hospitals for further evaluation.
The building was evacuated as a precaution.
Feola said that while emergency responders have not been able to determine what caused the students to faint, they have determined the building is safe.
"We believe that students who witnessed the first three children faint became anxious and panic ensued. We ask that you reassure your children this evening that this was an isolated incident and that they are safe when they return to the middle school tomorrow," Feola said in a statement.
Students were being bused home from the high school and will not be able to get back into the middle school this afternoon.
School will resume as scheduled on Friday.