Families from Our Lady's High and Taylor High in Motherwell demonstrated outside the headquarters of North Lanarkshire Council in Coatbridge.
The council is currently consulting on plans to close the two secondaries and build a new £36 million school on the former steelworks site at Ravenscraig, on the outskirts of the town.
The local authority believes the proposal is necessary because of deteriorating buildings at both Catholic schools. However, parents and some local politicians have opposed the plan, arguing the existing schools are in a reasonable condition and are vital to their communities.
They have accused the council of trying to use the development - which also involves special school Bothwellpark High - as a way of generating interest in the Ravenscraig redevelopment among housing developers and major retailers. And they do not want their children to go to a school built on a former industrial site where toxic waste was stored.
However, following detailed assessments, the council says the Ravenscraig site is ready to use and there are no post-industrial health and safety risks.
Catherine Brown, chairwoman of the Taylor High parent council, said the demonstration was held to try and get the council officials to the families' views.
"They are not listening to the many concerns that we have about the proposals and there is growing anger about that," she said. "We are concerned the Labour councillors are not getting behind the communities that voted them into power.
"These schools are at the heart of their communities and we believe they are using the plans for a new school to save the white elephant of Ravenscraig."
Jim Logue, the council's convener of learning and leisure services, said the proposed merger had the potential to bring together the best aspects of each school in buildings fit for the 21st century.