Students at Cowra High School were evacuated on Wednesday as a precaution after the school "received certain information from an anonymous source".
In a statement sent to students and their families, Cowra High School Principal, Helene Hamilton, said the evacuation was done in consultation with police.
"The evacuation proceeded in an orderly way and without incident," she said.
"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."
Ms Hamilton said the school was given the all clear from police to return to regular learning.
"After police attended the site and gave approval, students and staff returned to school and the school is safe to resume teaching and learning as of Thursday, October 29, 2020."
It is unclear if the incident is related to a number of hoax emails containing bomb threats which were sent to around 20 schools in NSW on Tuesday.
Wednesday's HSC biology exam was then interrupted after numerous schools received a threatening email, the NSW Education Standards Authority said.
Yass High School, Narrabri High School and schools on the south coast were among those affected on Wednesday.
Following the disruptions, Police Minister David Elliot said the Cyber Crimes Squad was now investigating the incidents.
"We won't allow you to interrupt the future of these thousands of young people, they have gone through too much in a very, very difficult year," he said.
"I'm delighted to see the work being done between the Department of Education and NSW Police to make sure that all contingencies are being dealt with.
"The largest cybercrime force in the world is now on your case."
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.
The main exams affected were legal studies and visual arts in the morning, but most afternoon exams went ahead as planned.
Ms Mitchell said any HSC student whose exam was disrupted by an evacuation would get special consideration when it was marked.