Two air ambulances were seen in the school grounds along with three road ambulances. There was also a heavy police presence with hundreds of parents waiting outside the school gates for their children.
The bilingual school has around 1,720 pupils aged between 11 and 18-years old.
A spokesperson for Dyfed Powys Police said: “Dyfed-Powys Police is dealing with an incident at Amman Valley School in Carmarthenshire.
“Three people have been injured and are receiving treatment.
“One person has been arrested and we are not looking for anyone else in connection with this incident.
“Emergency services remain at scene and the school has been closed while investigations continue.
“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.”
The school has released a statement saying the incident had been contained and that children were now being allowed to leave.
A spokesperson for the school said: “You will be aware of the incident that has taken place at Ysgol Dyffryn Aman today.
“The family members of all injured persons have been informed. We would like to reassure parents and the public that the incident has been contained.”
Reacting to the news, Local MP Jonathan Edwards said his “thoughts are with everyone back home and with the authorities responding.”
First Minister, Vaughan Gething said he was “shocked to hear of [the] serious incident” and that it was “a deeply worrying time for the school, families and community”.
Andrew RT Davies MS, Leader of the Welsh Conservatives, said: “The reports emerging from Ysgol Dyffryn Amman are extremely concerning.
“Schools should be a place of safety, learning and discovery. It is deeply sad and distressing that today teachers and pupils have had that safety shattered by a violent incident.
“My thoughts are with all staff and pupils at the school, and we thank the emergency services for their work.”
Adam Price, MS for Carmarthen East and Dinefwr, said: “Newyddion ofnadwy – yn meddwl am bawb sydd wedi eu heffeithio.
“Awful news – thinking of everyone who has been affected.”
