The MP for the area described the incident as something you would usually associate with "the States"
Officers were called to Milford Haven Comprehensive School in south-west Wales at about 3.20pm on Thursday following reports that a member of staff was assaulted by a pupil brandishing a weapon, not believed to be a knife.
The teacher was taken to hospital and discharged later in the evening.
A 15-year-old boy was arrested on suspicion of attempted murder and remains in police custody.
The school will remain shut on Friday.
Superintendent Chris Neve, of Dyfed-Powys Police, said: "We can confirm police are at Milford Haven Comprehensive School following a report of the assault of a teacher by a pupil brandishing a weapon at the school at approx 3.20pm.
"The teacher's injury is not a stab injury.
"A lockdown was implemented but has now been lifted.
"The teacher is receiving medical treatment for their injuries.
"All pupils at the location are safe and most have gone home. Officers remain at the school.
"A 15-year-old boy has been arrested on suspicion of attempted murder and is in police custody."
In a statement, a spokesperson for Pembrokeshire County Council confirmed police attended the school following "a report of the assault of a teacher by a pupil at the school".
"All pupils at the location are safe, and most have left the school. Police officers remain at the site. Any planned after-school events have been cancelled this evening," they said.
Mid and South Pembrokeshire MP Henry Tufnell added: "It’s a really tight-knit community in Pembrokeshire, everyone knows everyone else particularly in Milford Haven, it’s a really close community, and that’s why something like this is so upsetting.
"All the talk recently has been about the social media ban and the phones in schools and the importance of having kids focused in their classrooms.
"The violence in schools, normally it’s on one side of the Atlantic and that’s why it’s so upsetting to see this sort of violence especially in a local community like Milford Haven, which is so tight and close.
"Normally you read about these reports in, you know, in the States – you don’t hear about it in the UK so much."
Reacting to the news, Baroness Eluned Morgan, who visited the school only last week, said she was "deeply concerned by the incident" and that “violence of any kind has no place in our schools."
She said: 'I am deeply concerned by the incident at the school in Milford Haven. Having visited the school only last week, this news is particularly shocking.
"My thoughts are with the injured teacher, their family and the entire school community during this difficult time.“I want to pay tribute to the emergency services and school staff for their swift and professional response in ensuring the safety of pupils and staff.
"While this remains an active police investigation that must be allowed to proceed without interference, I want to be unequivocal. Violence of any kind has no place in our schools."
