Two reception-aged children scaled a 6ft-high fence to escape school and were found a mile away at their grandmother’s house.
Thirsk Community Primary School said the pupils were “quickly located” after a search of the area.
It is thought the children, aged four or five, would have also crossed a busy main road to get to Norby, where they were found on Friday afternoon.
North Yorkshire County Council said it has launched an investigation.
In a message to parents, head teacher Richard Chandler said the boys scaled the fence in the corner of the school field at the end of lunchtime.
He said: “As the class lined up to go back into the school, the two boys were not accounted for.
“An immediate search of the school and grounds was made and then the parents were called. The two boys were quickly located.”
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Mr Chandler said: “The school takes its responsibilities for pupil safety very seriously.”
He said the school has since reviewed its supervision arrangements and has implemented “several further measures”.
The children at the school have also been spoken to about the importance of staying safe, Mr Chandler added.