Messages posted on social media claimed the user planned to "kill as many people as possible" at Montgomery High School in Bispham on Monday, the Blackpool Gazette reported.
The planned attack was in revenge for alleged bullying, the post added.
Police confirmed officers were investigating the source and would ensure the school's safety.
The unnamed Facebook user also praised gun killers in schools in the United States and posted on the school's Facebook page: "Nobody talks to me or notices me except when they're calling me a nerd and pushing me around."
Responding to one student after the post was made, the user asked: "You think it's a joke? You'll see bullets, bodies, and blood."
Principal Tony Nicholson said in a statement to parents and carers the safety of pupils and staff was paramount.
It read: "Police have confidently reinforced the view that these online postings do not represent a credible threat. They, and we, wish to reassure our parents and the wider school community that Montgomery High School can open as normal on Monday morning."
A Lancashire Police spokesman said: "We are aware of posts on social media concerning Montgomery High School in Bispham and are investigating the source and content of these messages.
"If the matter is not resolved prior to Monday, parents, pupils, staff and the wider community should be reassured that an appropriate police response will be implemented prior to the school reopening after the Christmas holiday to ensure the safety of all."
Threats to kill pupils in a shooting at a Blackpool school are being investigated by Lancashire police.