A Leyland high school was put on lock down following a threatening phone call.
Children at Balshaw's CE High School, on Church Road, were ordered to hide under their desks as they took part in their final lessons of the day.
A Lancashire police spokesman said: "We were called at shortly after 3pm today to reports of a bomb threat being made to Balshaw’s Church of England High School.
"Officers attended and pupils were able to leave the premises without incident.
"At this stage we do not believe it to be a credible threat and are treating it as a hoax call."
An investigation is now underway to trace the caller.
One year seven pupil, who did not want to be named, said: "We were on the computers and I saw my teacher go out of the room. There was another teacher outside and my friend said to me she heard the word 'lock down'. He came back inside and said 'everybody got under the desks'.
"We got under the desks and put our chairs in front of us. When we did that, Sir turned the lights out and then he locked the door.
"He went back to his desk and he crouched down, he looked at his computer and he kept going to the door and looking down the corridor.
"Everybody in the class was looking at each other. We thought it was not real at first and then my friend said it was 35 minutes past.
"The bell normally goes at 3.30pm. I said 'I think it's real, because they wouldn't keep us back for so long'.
"After a while the head left a message on the speakers and he said all the students can now go home and leave the school quickly and quietly.
"We ran out the doors and there were police inside next to the door, and there was a police (officer) outside the door, and outside at the school gates, and the headmaster was outside the school gates."
The pupil said the lock-down began at around 3.20pm, and ended at 3.40pm.
"At first I was not sure what was going on because we had already done a practice on the lock down, so I thought it was just another practice," she said. "But as soon as my friend said it was past the time the bell would go I was getting really scared because I didn't know what would happen, and other people were getting scared."
An email from the school, shared with the Lancashire Post, read: "As a result of a phone call received, the decision was taken to put the school into its well-rehearsed lock down procedure. Police assisted and were in attendance. Following safety checks all pupils were released safely. Enquiries are ongoing to trace the offender."
The school was approached for comment.