Parents from Abronhill High, in Cumbernauld, believe court action is now the only way they can keep the school open.
Campaigners are currently trying to raise £1000 to start legal action against North Lanarkshire Council, claiming the authority has not fulfilled its obligations under 2010 school closure legislation.
A spokesman said: "We do not think the council has followed the correct procedures in documents produced after the consultation and we still have concerns over some of the figures quoted as potential savings.
"We have taken legal advice and we believe we have a good case and we intend to take this to court."
In February this year, North Lanarkshire's learning and leisure services committee approved the rationalisation of Abronhill and Cumbernauld High Schools.
After voting to close Abronhill, the council said the decision aimed to address the falling school roll and achieve annual savings of £1.3 million.
Under the plan, Abronhill will close by August 2014 in time for pupils to start their education at Cumbernauld High School, with the promise of a new purpose-built school to replace both campuses.
However, parents immediately appealed the decision to the Scottish Government, which called it in for review.
The government informed the authority in March that it had concerns over the educational benefit of the proposed closure and said the merger move may "lack clarity".
However, in June ministers approved the council's decision, albeit with a number of conditions covering aspects such as funding, the timetable for change and pupil safety issues.
Jim Logue, the council's convener of learning, said: "The schools are hugely under occupied and, as an educational authority, we have already seen the benefits of merging under-occupied secondary and primary schools and providing pupils with wider educational opportunities.
"To be clear, this is not solely about money, although we cannot continue to heat, light and maintain empty classrooms. This is about providing better opportunities to our young people now and for generations to come."
Many of the exterior scenes for Gregory's Girl, which starred John Gordon Sinclair as Gregory, Dee Hepburn as Dorothy and Clare Grogan as Susan, were filmed outside Abronhill High.
Meanwhile, campaigners fighting to save the only Catholic primary school in a suburb of Glasgow will take their campaign to the Liberal Democrat Conference in Glasgow today.
Parents, children and supporters from St Joseph's plan to lobby MPs and MSPs as they arrive at the SECC in Glasgow for the start of the conference. East Dunbartonshire MP Jo Swinson is due to speak at the event.
Liberal Democrat-led East Dunbartonshire Council is currently consulting on proposals to close St Joseph's, which has been in Milngavie for well over a century and educate pupils in Bearsden.
Helen Williams, vice-chairwoman of St Joseph's parent council, said: "We decided to lobby the conference because we want our MP Jo Swinson and our local councillor Eric Gotts to stand up for our community. St Joseph's is the fastest-growing school in East Dunbartonshire.
"It is unbelievable that we are being targeted for closure when our roll has increased by 26% in the past three years."