Pupils were locked into their school in Cardiff and police called after fears a man would take a child from the premises.
Officers went to St Teilo’s Church in Wales High School in Llanederyn after reports relating to the welfare of a child, South Wales Police said.
Staff at the 1,300-pupil school locked the gates to prevent the child being taken away from the premises by a man known to him and police were alerted just after 3pm on Thursday November 7, the force confirmed.
It is understood no one was able to leave or enter the school site in Circle Way East.
A South Wales Police spokeswoman said: “Officers responded promptly to the report which required dialogue, compassion and patience to ensure it was resolved in the most appropriate manner.
“We were satisfied there was no threat to the wider school community, however, pupils remained on site while the situation was resolved.
“The pupil concerned was subsequently safeguarded and a man, who is known to the child, was detained under the Mental Health Act.
“We would like to thank those affected for their patience and understanding during the incident.”
St Teilo’s Church-in-Wales High School is a mixed 11 to 18 comprehensive voluntary-aided faith school ranked good at its last Estyn inspection in 2014.
A spokeswoman for the school said: "An event took place at St Teilo’s School yesterday which required staff to close the school site for a short time. Whilst this caused delays to the school buses, there was at no time a risk to children.”
