Police are hunting for a group of Year 12 boys who allegedly chased and attacked students from St Luke's Grammar School, Dee Why, as they got off a school bus on Tuesday afternoon.

The younger boys were targeted just after 4pm as they walked home from the bus stop near Wyadra Avenue, Freshwater.

A Year 7 student ran away to nearby Harbord Park where he was allegedly tackled to the ground by a male who then fled the scene, a police spokesman said.

Primary school parents were sent this letter from the principal. Photo: Supplied

The 13-year-old suffered a broken leg and was taken by ambulance to Royal North Shore Hospital where he had surgery on Wednesday.

His injuries were described as very serious.

An 18-year-old, from Allambie Heights, handed himself in to Dee Why Police Station on Wednesday afternoon.

He was arrested and charged with reckless wounding causing grievous bodily harm. He was released on bail and is due to appear in Manly Local Court on October 12.

Northern Beaches police also urged others involved to hand themselves in.

The group of Year 12 students have not been identified but are believed to be from northern beaches Catholic schools close to St Luke's.

A spokeswoman for the Catholic Schools Office, Diocese of Broken Bay, said she didn't know which schools were involved and had been told it was in the hands of police.

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In a letter sent home to parents of St Luke's students, principal Jann Robinson said the alleged attackers were "Year 12 boys from some other local schools".

She said the student, who Fairfax Media has decided not to identify, was "badly hurt".

"Our thoughts and prayer are with the [boy's] family at this time," she said.

A police spokesman said: "Investigations are continuing and police are urging anyone with information to come forward," a spokesman said.

Year 12 students in NSW finished the school year on Tuesday and will start their HSC exams on October 13.

In recent years, schools have banned muck-up days or replaced them with other activities with following violent and inappropriate incidents.