Police requested the schools be locked in to deal as a precaution with an unspecified incident.
The incident was deemed over and the lock in lifted.
The police activity was confined to Jo Mar Drive, and one person is reportedly in custody.
Nowtown High School initially tweeted: "All children safe. NHS is in LOCK-IN CLOSED. Precautionary - advised by NPD. #newtownhs"
Then, just before noon, the following tweet was sent: "All children safe. NHS is NO LONGER in Lock-In Closed. NPD has informed schools that threat no longer exists. #newtownhs"