Thirteen children were involved in the incident which happened at around 9am at Minster College on the Isle of Sheppey.
One of the most seriously injured children has been flown to hospital by a rescue helicopter.
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Kent Fire and Rescue Service (KFRS) were called to the scene.
A spokesperson from the KFRS said: "KFRS was called at 9.18am to reports that a heating duct in a sports hall had collapsed. It sent three appliances plus its specialist Urban Search and Rescue team to the scene but no-one was trapped.
"Crews initially helped with first aid and to make the scene safe and KFRS operations finished at 10.09am but firefighters are on standby to help colleagues from Kent Police, South East Coast Ambulance NHS Trust and the Kent Air Ambulance who are also in attendance."
The school has been closed for the day.
(CD/JM)
