Police believe a shot was fired into a school in Sydney's south-west about lunchtime on Tuesday.
Police were called to Macquarie Fields Public School at 12.45pm after reports a "window had been broken by a metal projectile".
Students and staff were uninjured and the school was not forced into lockdown, police said.
Superintendent Grant Healy said a teacher had alerted police after the object had smashed a glass window, stopped by a curtain. There were Year one and two students were inside that classroom.
"This was not a targeted act towards a school," he said.
"It was established the projectile may have been a bullet and early and early investigations reveal the projectile may have come from some distance away."
Preliminary investigations have revealed the object is a small calibre bullet, which specialist police have seized and will forensically examine.
"Early investigations reveal the projectile may have come from some distance away," said Campbelltown area commander Detective Superintendent Grant Healey.
"It's a totally reckless act by somebody who's acting in a way, with a firearm, that's very dangerous to the community."
Inquiries are continuing. Anyone with information is asked to contact Campbelltown Police or Crime Stoppers on 1800 333 000.
