Emergency services were called to the scene outside a school on Langford Drive at Kariong, west of Gosford about 11pm on Tuesday, where they found the 13-year-old had been stabbed in the chest.

People at the scene performed CPR on the teen but were unable to save him.

He was taken to Gosford Hospital in a critical condition but died a short time later.

A 13-year-old boy has been arrested and charged with murder and is due to face a children's court on Wednesday.

Police seized a kitchen knife believed to be the murder weapon.

Brisbane Water Police District Commander Detective Superintendent Darryl Jobson said the stabbing was a "tragic incident" and "the ripple effects will go through the whole community".

The two boys knew one another and police are looking into whether they had arranged to meet up and fight.

Det Supt Jobson says police have identified six witnesses but believe there are more who were yet to be identified and called on them to come forward.

"It's likely that there's young kids that are going to be witnesses," Det Supt Jobson says.

"This is an incident of tragic proportions and the lives of many have changed forever. As you would expect all the families involved in this are upset. It's a traumatic experience."

Australian Associated Press