A supply teacher is claimed to have pupils as young as 11 to "go home, get metal cans and cut yourselves."
The disturbing outburst is believed to have taken place at Maricourt Catholic High School in Maghull, Merseyside, on Wednesday.
The teacher is said to have made the comment to four boys, and the school informed their parents.
The concerned parents of all four boys are said to have been "astonished and angry" with the revelation and went to to the school to speak to the head.
It is believed that one parent, after discovering the news, tried to confront the teacher in the school corridor.
The head summoned the teacher, an agency worker, to his office but the Liverpool Echo reports he chose not to comment and was asked to leave the premises.
A parent, who has a child at the school but not one of the boys concerned, said: "Who is this teacher? Has he been properly vetted?
"There are so many questions that need answering.
"The school acted without hesitation and removed the teacher while they looked into this.
"They have acted completely properly.
"Nobody really knows why this teacher said this, I wonder if he was getting exasperated with the children at the time? It's bizarre."
It is understood the comment was made at the end of a lesson in a classroom.
A spokesperson for Maricourt Catholic High said: "We expect the highest standards from all in our school community, not least our dedicated staff, whether they are permanent or temporary.
"We received reports that a supply teacher made comments to pupils, which fell short of our high standards.
"The supply agency has been made aware of the teacher’s conduct and the teacher has not been used by the school while investigations are ongoing."
