Eleven St. Michael the Archangel Middle School students and one adult were taken to Lehigh Valley Hospital Wednesday morning after they began feeling ill.
The school -- on Old Bethlehem Pike in Upper Saucon Township -- was evacuated and classes dismissed for the day.
Emergency crews checked to see if gas or carbon monoxide might have been the problem but they found no evidence of a leak in the school or Assumption of the Blessed Virgin Mary Church.
Officials now say a "heat-related issue" may be the reason that the students and adult felt lightheaded and nauseous after they went from school into the church to attend mass.
Five students passed out and were unconscious briefly.
Twenty-seven units were called to the scene around 9:30 a.m.
St. Michael the Archangel Middle School, which enrolls students from throughout Lehigh County, is in the Colesville section of the township on the property at Assumption BVM Church and enrolls students in grades five through eight. The middle and primary school enroll about 320 students from 198 families, according to the school's website.
The primary school is at 5040 St. Joseph's Road, also in the township.