A 16-year-old boy was impaled through the side with a goal post at a school sports field in Essex, police said.
Fire crews were called to Beauchamps High School, in Wickford, following the incident, described as a "terrible accident" by head teacher Philip Bell.
The victim was taken to Basildon hospital where his condition is described as stable.
A 16-year-old boy has been questioned and released by Essex Police who now believe it was an accident.
The incident involved a plastic pole with a metal tip used for temporary goal posts.
Safety regulations
Firefighters used hydraulic cutting equipment to shorten the temporary goal post, following the incident at 1020 GMT.
Speaking soon after the event, Mr Bell said: "A boy has been injured on school grounds in a terrible accident.
"The school has taken prompt action, his parents were informed immediately and are now at the school and we are working with the emergency services to get the boy to hospital as quickly as possible.
"A full investigation into how this accident happened will take place."
Essex County Council added: "We are working with the school to ensure the pupils and staff affected by this distressing incident receive the support they need.
"We await the outcome of the school and police investigation, but are confident staff at Beauchamps High will have been adhering to national health and safety regulations."
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