Up to 30 pellets were fired from an air rifle into the playground of a County Antrim school, police have said.
Three people, including a 13-year-old boy, arrested after the shooting at Dunclug College, Ballymena, were released pending further inquiries.
Police have ruled out a sectarian motive for the shooting which occurred at about 1310 BST on Tuesday.
An air rifle and a gas-propelled pellet gun were recovered after police searched a number of properties.
A boy was later arrested on suspicion of firearms offences. Two males, aged 17 and 40, were also arrested but police said all three were later released pending further inquiries.
Dunclug principal Ruth Wilson said on Tuesday that at least one metal pellet had struck the door of a gym during the incident.
She said about 30 or 40 pupils were in the playground at the time and had to be moved elsewhere.
"There were around 30 or 40 children out during 'junior' lunch involving mostly 12-year-olds. Most of the pupils were still inside getting their lunch," she said.
"We immediately cleared the playground without panic and thankfully, to our knowledge, no one was hurt. The children were moved to the other playground and police were called straight away.
"We believe the pellet was fired from outside the school fence from the direction of the Doury Road estate. There is no evidence to suggest they were firing towards any child."
She said that none of the children had to go home after the incident and there was no trauma.
"At our regular Wednesday assembly we will mention this tomorrow to reassure people that no one was hurt," said Mrs Wilson.