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Officers from the Health and Safety Executive (HSE) are continuing to investigate after part of Minster College collapsed while pupils were sitting an exam.
Head teacher Alan Klee confirmed all but one of the 12 pupils who were rushed to hospital after a section of heating duct caved in on Tuesday are now at home.
One male student has undergone surgery for a fractured cheekbone and is still at Medway Maritime Hospital.
Mr Klee said: "One pupil is out of surgery now following an operation for a fractured cheekbone. Everyone else is out of hospital."
~ How the drama unfolded - in words, pictures, audio and video >>>
Year 9 pupils had just begun a SAT test when the 100ft-length of heating duct fell 20ft to the floor. Because of the way the desks had been laid out, the majority of the unit landed in between the seated pupils.
A spokesman for the HSE said: "We currently have inspectors on site including a specialist construction engineering inspector.
"We are working with the school and Kent County Council to ascertain the cause of the incident and see if there are any lessons to be learnt. We can confirm that there was no asbestos present in the heating duct or the hall."