Around 100 people attended a protest calling for the completion of the new 16-classroom and two-unit special needs facility at the Holy Family School in Mullingar.
The new school project began in September 2017 in what was meant to be a 12-month construction programme.
Westmeath County Council is managing the project under a devolved service level agreement with the Department of Education and Skills.
Parents and pupils gathered in town's Market Square, before marching to council buildings to demand a completion date for the school.
They also claimed that Curraghmore NS has had to freeze its enrollment process for next September, which has put extra pressure for school places on all schools in Mullingar Parish.
In a statement, Westmeath County Council said all classrooms are now close to completion and have had the final coat of paint applied.
It said works are progressing in the circulation areas, ancillary rooms, the special needs unit, as well as on external works, floors and ceilings.
The council said that the "speed of completion is primarily related to the level of resources employed on-site and while numbers on site have increased slightly over the past week, the level of resources and the rate of progress on-site remains a concern".