KINETON High School went into lockdown and police were called after three masked youths entered the grounds.
Officers were called at just after 10.15am on Wednesday (April 19) and pupils were kept inside the school and told to hide under their desks as a precaution while the intruders were traced.
Parents have reported receiving frightened text messages from their children as they hid and teachers barricaded the doors to the classrooms.
Warwickshire Police said this was an isolated incident relating to local youths, which was contained and has been safely resolved.
There is no wider risk to the pupils or teachers who are all safe and well, the force added.
No offences have been established currently, however enquiries are ongoing.
Kineton High School wrote on its Facebook page following the incident: “There was an incident in school today which involved a call to the police shortly before 10.15am. Please be assured that the incident was contained, is over and all pupils and staff at the school are safe.
“The school’s lockdown procedure was deployed as a precautionary measure as three unidentified young people were on site. Police are remaining in the area to reassure the public.
“This was an isolated incident and school has returned to its normal activities. The school remains fully open for the education of all its students. The safety and welfare of students is its most important priority.”