Police are questioning a 13-year-old boy after a student was stabbed in a classroom on the south coast.

The 14-year-old victim was stabbed in the chest about midday on Monday at Shoalhaven High School in Nowra.

Police arrested another 14-year-old boy in the classroom but later released him.

They said a 13-year-old boy later arrived at Nowra Police Station with his mother and was helping officers with their inquiries.

Inspector John Behrendt from Nowra police said the victim was stabbed in the left side of his chest. He said the stabbing was believed to have happened in the classroom.

The victim was taken to Shoalhaven Hospital then transferred to Sydney's Children Hospital in a serious condition. He has since improved to be in a stable condition.