NSW Premier Gladys Berejiklian confirmed a person rang the school on Thursday morning saying there was a bomb, forcing the school to evacuate before HSC students were due to sit their modern history exam.

Ms Berejiklian was briefed by Police Commissioner Mick Fuller and told earlier bomb hoaxes emailed to NSW schools on Tuesday and Wednesday had been sourced to eastern Europe.

"He's allowed me to confirm that the initial email threats actually emanated from eastern Europe," she said.

The threatening emails were sent to around 20 Sydney schools on Tuesday and some regional schools were evacuated on Wednesday, causing thousands of HSC students to abandon their exam papers as schools were evacuated.

NSW Police are working with Australian Federal Police and federal authorities on the matter.

"I have confidence that the police and the education department will resolve what is in the best interest of every student," Ms Berejilkian said.

"At the end of the day what matters most is safety and wellbeing of the students."

The NSW Education Standards Authority said no affected student would have to re-sit an exam and they would be given special consideration by markers.

Australian Associated Press