A boy held over the stabbing of a teacher in Tewkesbury was arrested on suspicion of attempted murder, police say.
Armed police arrested the teenage boy an hour after the stabbing.
The motivation of the attack is unclear. Police say the teacher is in a stable condition.
A knife was seized at the time of the attack.
Tewkesbury school, in Ashchurch Road, was put into lockdown following the incident
Officers were called at about 9.10am on Monday following a report that a pupil had stabbed a teacher.
In a statement released to parents and carers earlier, headteacher Kathleen McGillycuddy said: “We appreciate this morning has been a worrying time for all associated with Tewkesbury Academy.”
She added: “We were alerted shortly before 9am this morning to reports of a serious incident which left a male member of staff in need of hospital treatment.
“The school was locked down and police were called immediately when we became aware of this incident.”