Students at a Sydney high school will have to re-sit HSC trial exams after the test papers appeared ahead of time.

The Catholic Secondary Schools Association (CSSA) provides HSC trial exam papers to about 480 government, Catholic and independent schools.

The CSSA has announced it is aware of a security breach at an unnamed high school, which it will not name because the breach did not appear to involve staff or school procedures.

CSSA spokesperson Jim Hanna said the breach affected three exams for one class.

“When that became known the school took immediate action to protect the integrity of the exam and its assessment process,” Mr Hanna said.

Local police were notified and an investigation launched, which ended when a male student handed himself in to the school.

Police are still investigating a separate incident at Sydney's exclusive Scots College, in which it is alleged that two physics exam papers were stolen during a break-in the night before the trial exam.