Youngsters at Our Lady and St Hubert's RC Junior School in Harwood Lane, Great Harwood, will return to class a day late after delays due to building work.
Ongoing renovation at the school is seeing the bathrooms extended as well as work on classrooms, the headteacher's office and foyer.
Delays in finishing the bathrooms mean pupils must start term on Wednesday.
Headteacher Kay Schofield said she had been left "frustrated" by the delays.
She said: "The work has been taking place throughout the summer and unfortunately due to heath and safety reasons the school is not ready for the junior pupils."
The school has been attempting to notify parents through letters and phone calls. The infant school is unaffected by building work.
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