Dozens of student were sickened at Young Women's Preparatory Academy in southeastern Houston because of a noxious smell to Thursday morning that was later determined to be hot tar being used for construction.
The source was not a gas leak, as many had suspected and had been earlier reported.
The Houston Fire Department says 51 people complained of headaches and nausea -- and some students were taken to area hospitals as a precaution.
Ambulances and EMS personnel -- as well as the Houston Fire Department's bus-sized Mass Casualty Evacuation Ambulance -- were at the scene by lunchtime.
The school closed early for the day and students were picked up by parents.