Speaking to the Bristol Post, Jennifer Ryan says her son Dyllon, 13, was twice threatened with a blade by a fellow pupil at Marlwood School near Bristol.
She said her family have been left angry that staff failed to take immediate action against the girl who allegedly threatened the teenager.
Jennifer, 36, says Dyllon, 13, was confronted by an older female pupil who held a craft knife to his throat after he reported her over an incident with an aerosol can.
The outraged mum told the paper that staff failed to immediately exclude the pupil and delayed calling the police to report that Dyllon had been threatened.
She said the first incident happened last Monday when Dyllon - a Year Eight pupil - and another student told teachers the Year Nine girl had sprayed an aerosol can in the school and lit the fumes.
Mrs Ryan, a practice nurse, says that the girl then approached her son and threatened him with a knife she is believed to have taken from the art block. She later had the knife confiscated.
It is then claimed that later the same day, the same girl burst in to a classroom and grabbed Dyllon, who has dyslexia and ADHD, before pinning him to the wall and putting a knife to his throat.
Although this was witnessed by other pupils and teachers, Mrs Ryan says the school decided not to make contact with her or report the assault to the police.
"Why was this girl not excluded in the morning from her already-dangerous stunt? Why was nothing done following the first assault with a knife and why after a second assault with a knife were we not contacted at any point in time?" the mum-of-five asked.
"Dyllon says she shouted out that she was going to 'shank him'. It is totally disgusting that we should have to find this information from our son when he arrives home traumatised from school."
Mrs Ryan said Dyllon was left so upset that he had not been able to return to school.
"When I got home on Monday he looked like he had been crying and was shaken up," she said. "Dyllon has been bullied before and is very vulnerable. When I heard what had taken place I couldn't believe what had happened.
"I understand the girl concerned has now been permanently excluded but shouldn't have taken such a serious incident to have brought it about.
"She could have been excluded after the incident with the aerosol can and then after she had made the first threat with the knife."
A spokesman for South Gloucestershire Council issued a statement on behalf of the school, saying: "The school is working with the police in relation to an incident that took place on Monday.
"The incident is the subject of a police investigation and we are unable to comment any further at this time."
A police spokesperson for Avon and Somerset confirmed they were investigating after receiving a report of an assault at Marlwood School.
"The incident, which happened on Monday (25/11), involved two pupils both aged 13," the spokesperson said. "A boy suffered a minor neck injury as a result. Enquiries are continuing and we are working closely with the school to progress the investigation."