A pupil described as "happy and popular" by his headmaster stabbed himself in the stomach in front of his shocked classmates at a school in Exeter earlier this week.
The 13-year-old boy, who cannot be named, was rushed to hospital for surgery after wounding himself with a four-inch kitchen knife at Dawlish Community College.
At the time of going to press, the boy was stable and conscious and his injuries were not considered life-threatening.
Andrew Davis, headteacher of the school, described the incident as "tragic" and said it had left staff and pupils shocked.
Police said they are not treating the incident as suspicious. No other pupils were involved.
Mr Davis said the boy's actions had come completely out of the blue. "We are quite a small school and I know the pupil concerned well," he said. "He is a popular student and we had no indication of anything untoward.
"I saw him out on the field playing football and smiling away with lots of other people. He is not somebody we had any concerns about, which is why this incident was completely unforeseen."
About 20 pupils witnessed the incident and 10 were so upset they were sent home. Others received counselling at the school.
Mr Davis added: "Our immediate and paramount concern is the welfare of the injured student and those who witnessed the incident.
"They were all cared for by staff, including our school counsellor. Occurrences such as this are rare and it has hit both staff and students very hard, but I would like to praise the positive way staff and students involved responded."
Professor Robert Burden, a psychologist specialising in children and young people at Exeter University, said incidents of young people harming themselves were becoming more frequent.
He said: "The pressures on young people are growing and the provision of therapeutic support is much more of a premium these days."