Police were called to a school in Leeds after a major disturbance broke out among pupils.
One person was arrested on suspicion of criminal damage when officers attended South Leeds High on Middleton Road at Belle Isle on Wednesday afternoon.
Head teacher Colin Bell said it was an isolated incident caused by four youths who were not pupils at the school.
Nobody was hurt and students returned to school for their afternoon lessons. A police investigation is under way.
A 16-year-old boy was later given a fixed penalty notice for a public order offence.
Saddened by threat
Neither he, nor the person arrested at the scene, were pupils at the school, police said.
Mr Bell said the police were called to ensure the safety of staff and students and stayed on site until students had returned home safely.
"South Leeds High School prides itself on being a happy and safe environment, where students and staff work closely together and make a positive contribution to their community," he said.
"I'm saddened it seems that there are individuals external to the school who wish to threaten this.
"I would like to emphasise that this was an isolated incident and would call upon the school and its wider community to remain calm, behave maturely and not react to any rumours which may circulate."