By Kim Glovas, Natasha Brown

POTTSTOWN, Pa. (CBS) –- The Pottsgrove School District says three students fainted this morning during choral practice and other students may have overreacted.

Pottsgrove School District superintendent Shelley Feola says the incident began about 10am at Pottsgrove Middle School, located on North Hanover Street in Pottstown. As additional students complained of similar symptoms, the school was evacuated.

Emergency personnel responded immediately to assist in the evacuation.  In addition, fire officials and a hazmat team were called to the scene.  Twenty-one students were taken to Pottstown Hospital for evaluation.

The rest of the students were moved to the high school as a precaution.

Twenty of the 21 students sent to the hospital were treated and released with one still being observed as of Thursday evening, says officials.

The superintendent said all students would be released today at their regularly scheduled bus time. School will reopen tomorrow as scheduled.

Feola says students will not be allowed to return to the middle school today to retrieve belongings.

The initial call to authorities was placed as a possible carbon monoxide leak.  The incident remains under investigation.