Merriwa Central School, Muswellbrook High School and Singleton High School were among the more than 10 schools in regional NSW targeted.

The NSW Education Standards Authority ( NESA) said Wednesday morning's HSC biology exam was interrupted after numerous schools received a threatening email.

They follow hoax emails containing bomb threats sent to about 20 NSW high schools on Tuesday, which led to thousands of Sydney HSC students having to abandon exams as schools were evacuated.

Muswellbrook High School released the following statement: "As you may be aware, students and staff at our school were evacuated as a precaution this afternoon.

"This was done in consultation with police after the school received certain information from an anonymous source.

"The evacuation proceeded in an orderly way and without incident. At no time was any student or staff member in danger.

"To ensure the wellbeing of students and staff, all NSW public schools have such procedures, which are practised regularly.

"After police attended the site and gave approval, students and staff returned to a normal routine."

If an exam was interrupted for 40 minutes or more at a school, students would be dismissed and would not have to re-sit the exam, NESA said.

If an exam was delayed by an hour or more it would also be abandoned and affected students would not do the exam.

The affected schools would submit group applications on behalf of their students asking for special consideration for illness and misadventure.

Education Minister Sarah Mitchell said the stunt was "a disgrace, not a funny prank" and warned severe penalties would be applied when the culprit was caught.

Ms Mitchell said any HSC student whose exam was disrupted by an evacuation would get special consideration when it was marked.

Australian Associated Press