Police said three people have been injured in an incident at Amman Valley School in Ammanford, Carmarthenshire on Wednesday.
One person has been arrested and the force said it is not looking for anyone else in relation to the incident.
Dyfed-Powys Police said the three injured people were receiving treatment and emergency services remain at the scene, while the school has been closed.
In a statement, the force said: “We are working closely with the school and Carmarthenshire County Council.
“We are aware that there is footage of the incident currently circulating on social media. We would ask that this is removed to avoid contempt of court and distress to those affected.
“We would ask the people do not speculate while there is a police investigation ongoing.”
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