A teenager was arrested after he was caught on school grounds in Port Lincoln on Wednesday morning.
Just before 12.45am, police responded to an alarm activation at a school on Lincoln Place.
Police heard noises coming from inside one of the buildings and quickly set up cordons before a teen exited and attempted to run from police.
The boy was caught by waiting patrols and arrested.
The 16-year-old Port Lincoln boy was charged with aggravated serious criminal trespass and was bailed to appear in the youth court on January 19.
Police suspect a second youth may have also been involved and investigations are continuing.