Except when the letters are written by the long arm of the law.

Cranbourne police have put schools on notice about students roaming at large during school hours, as well as writing to parents and guardians about children out after dark.

About 30 letters have been sent to families in the past three months after police have found children as young as 10 out late at night.

The letters state the time and location where patrolling police have approached the children.

Leading Senior Constable Brett Owen, a Casey police youth resource officer, said the initiative had received positive feedback and evoked surprise from many of the letters' recipients.

"Parents in some circumstances haven't known their children are out and about. Some say they were told they were staying at a friend's place, or believed they were in bed and snuck out."

The initiative started with a desire to cut down on the number of young people roaming at late hours in streets and transport hubs.

"These young people aren't being charged with anything but we consider it inappropriate for them to be out at that time.

"We want to prevent anti-social behaviour but it's also a welfare issue - particularly at those late hours, they could be committing crime or being a victim of crime."

Leading Senior Constable Owen advised parents to have a conversation with their straying children.

"If there are any behaviours or attitudes that need addressing, we recommend [parents] contact Casey Youth Services."