It is thought children who fell ill on a school bus were suffering the effects of diesel fumes.
Ms Sims: "No one is in a life-threatening condition"
Forty-two were treated for suspected carbon monoxide poisoning and 24 sent to hospital after several pupils complained of dizziness and nausea.
Emergency services declared a major incident after being alerted by Harris School in Rugby, Warwickshire.
A spokesman for Stagecoach, which runs the service, said no defects were found on the bus during recent checks.
They added that the firm was checking whether the exhaust system was damaged by an object while en route.
"As soon as we were made aware of a potential problem with the vehicle, the bus was stopped by the driver and immediately removed from service.
"We are already working closely with the police and other authorities as part of the investigation into the incident."
In a statement the school said: "We are advised the most likely cause of their discomfort was irritation from diesel fumes.
"Most student returned to class on the advice of ambulance staff.
Hospital on stand by
"The school is grateful to all support services who attended and assisted us."
Warwickshire Police and the Health and Safety Executive have opened and investigation.
A spokesman for West Midlands Ambulance Service said they declared a major incident and despatched five ambulances, an emergency planning officer and alerted nearby Walsgrave Hospital to expect patients.
The voluntarily-aided Church of England school teaches nearly 700 pupils aged between 11 and 16.