Police were posted to Montgomery High School in Bispham after threats were made towards a pupil.
In a statement posted to the school's website today, executive principal Sean Bullen and headteacher Steve Careless said the threat was made a "few weeks ago" and was serious enough to call in police.
"Some of you may have noticed a police presence on the gates of Montgomery," it said.
"A few weeks ago, an individual who is not part of our school was threatening towards a Montgomery student.
"A number of Montgomery staff supported the student.
"Therefore, the school contacted the authorities for increased support while this issue persists.
"It relates to this one individual.
"Unfortunately all schools at times have issues like this, with individuals who are not part of the school community.
"Some parents have been wanting clarification, so this is it."
A spokesman for the Fylde Coast Academy Trust (FCAT), which runs the school, did not elaborate on the threat, but said it was "not uncommon" to see police - believed to be a Police Community Support Officer (PCSO) in this case - outside schools.
In early 2016, children returning to classes at Montgomery after the Christmas break did so under police guard after anonymous online threats, which pledged to carry out a US-style shooting at the school, in All Hallows Road.
One post said: "You think it's a joke? You'll see bullets, bodies, and blood," while another pledged to kill as many people as possible.
Although police said from the start the threat was not being treated as credible, initial attendances plummeted to just 268 - down from its usual 1,370 - with eight out of every 10 seats left empty on the first day back.
A Blackpool man was arrested in connection with the threats but was later released without charge.
There was no suggestion both threats are linked.