An investigation has been launched into allegations of HSC students plagiarising essays and parents bullying teachers into increasing the marks of their children at a Sydney private school.

The revelations are the latest in an unfolding plagiarism scandal across NSW, as the Board of Studies formally launches its own investigation into "endemic" cheating across the state's schools.

Pittwater House School, in Collaroy, where an inquiry into malpractice is under way. Credit:Lee Besford

Pittwater House school in Sydney's northern beaches announced on Wednesday it will investigate each school department after allegations were raised over malpractice.