Skerton High School will close on August 31, Lancashire County Council has announced.
Cabinet member for children, young people and schools, County Coun Matthew Tomlinson, signed off the plans on Thursday April 3.
The decision marks the end of a long period of consultation and discussion with the local community, many of whom wanted the school to stay open.
County Coun Tomlinson said: “This is a difficult and sad decision to have to make. I certainly didn’t come into this role to close schools and would not be doing so now if the arguments for closure were not so overwhelming.
“We cannot justify keeping open a school where students’ results are the worst in the country, and the school is in the bottom 10 per cent for helping students to improve.
“Many students are struggling when they start at Skerton and they are still struggling when they leave.
“I know that people have campaigned hard in the past year for Skerton to stay open, but the truth is that the school’s difficulties go back many years and things haven’t improved despite the best efforts of governors, the county council and other local schools.
“Ofsted inspectors have been very concerned about the school for many years and numbers of pupils have dwindled over at least a ten-year period.
“The sad but undeniable fact is that if we did not close Skerton, the Department for Education would almost certainly compel us to do so.
“I am very conscious of the distress and disruption any school closure causes for current pupils and we will do our utmost to help in any way we can. But we cannot continue to support a school which does not meet required standards, while costing significantly more per pupil than other schools which do.”