“All pupils at the location are safe and most have gone home,” said Supt Neve, adding that the teacher is receiving medical treatment for their injuries.
He added that officers remained at the school, and that the 15-year-old boy was in police custody.
A Welsh Ambulance Service spokesperson said they were called to the scene and one patient was transported to hospital.
Pembrokeshire council said most pupils had now left the school and confirmed police officers remained on site.
“Any planned after-school events have been cancelled this evening,” a spokesperson said.
Wales First Minister Eluned Morgan said her thoughts were with the injured teacher, their family and the entire school community.
“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,” she said.
Mid and South Pembrokeshire MP Henry Tufnell said he was “very grateful to the school staff and our emergency services for their swift response in ensuring the safety of pupils and colleagues”.