A high school on the lower north shore was the first to receive threats about 11am on Tuesday, via an anonymous email.

Since then, police say at least another 18 high schools across Sydney received similar threatening emails.

The schools all enacted their emergency response plans, evacuated students, and contacted police.

Local and specialist officers are searching the schools, but no items of interest had been found by the end of the school day, police say.

The threats caused thousands of students sitting their HSC exams to be pulled from classrooms on one of the most important days of their schooling lives.

Many shattered students said they didn't get to finish their tests.

"A lot of people's initial reaction was 'this can't be real'," Castle Hill High year 12 student Josh Gatt told Nine Newspapers.

"Most people are worried about how it will be marked."

Police believe the threats are linked and the Cybercrime Squad is investigating the origins of the emails.

"Police are reminding the community that sending threatening emails is a serious criminal offence," police said in a statement.

The NSW Educations Standards Authority said it was working with the schools to determine the impact on HSC exams.

"Schools will submit to NESA group applications for illness and misadventure on behalf of impacted students," a spokeswoman said.

"No student will have to re-sit an exam."

NSW Education confirmed more than 20 school received threats, but all the campuses would resume normal operations on Wednesday.

Australian Associated Press