It happened around 2:40 p.m.
According to officials, 11 children and 1 teacher's aide were treated for heat stress - this is the least severe of the most common heat illnesses including exhaustion and stroke.
Symptoms are lethargy, confusion and nausea.
Five people were taken to area hospitals for treatment.
The children - 120 of them, plus their teachers - were in the auditorium rehearsing for a concert, but the room was too hot and too many people in it, officials said.
A Heat Advisory was issued for Long Island on Thursday, with temperatures climbing near 100 degrees
"The combination of heat and humidity can become unbearable and dangerous for many of our residents, and the county is doing all it can to provide some relief," said County Executive Mangano. "I encourage everyone to take precautions to stay as cool as possible.
Cooling centers are open in Nassau County:
The Rockville Centre Recreation Center
111 Oceanside Road, Rockville Centre
Expected Hours of Operation: 10am ? 6pm
The Leonard S. Sandel Senior Center
50 South Park Avenue, Rockville Centre, New York
Town of Oyster Bay
Hicksville Community Center
28 West Carl Street, Hicksville, NY
Town of North Hempstead
Expected Hours of Operation: 11am-10pm
Clinton G. Martin Park
Marcus Avenue & New Hyde Park Rd., New Hyde Park
Michael J. Tully Park
1801 Evergreen Avenue, New Hyde Park
The Nassau County Department of Health also offers the following information for residents to protect themselves and others during high temperatures:
- Check on relatives and friends, especially the elderly.
- Increase time spent in an air-conditioned environment.
- Increase non-alcoholic fluid intake.
- Take cool baths.
- Individuals especially at risk, such as the elderly or those with chronic illnesses should refrain from physical activity, avoid being alone, keep their sleeping area well ventilated, and avoid falling asleep in a hot room.
- Eat light, but nutritious foods.
- Avoid the sun at the hottest times of the day.
- Wear a hat and light, loose-fitting clothing.
Elderly people, infants, and many individuals with chronic medical conditions are at high risk for heat-related problems and should be very cautious about exposure.
If any person experiences heat stroke or exhaustion, get medical help immediately.
Symptoms of heat-related illnesses may include: headache, dizziness, fatigue, anxiety, nausea, vomiting, and low pulse rate. Other symptoms may include cold pale and clammy skin or hot, flushed dry skin.
For more information, please contact the Nassau County Office of Emergency Management at (516) 573-0636.