Parents at a NSW public school say they have been left "horrified" after students were repeatedly placed in scripture classes against their parents' wishes and told they needed to have an interview with a deputy principal before they could attend non-scripture classes.

The NSW Department of Education is making inquiries into a letter sent to parents by the principal of Maclean High School in northern NSW, which strongly advocates for scripture classes and appears to breach the department's policy on religious education in several instances.

Parents also said students with written permission to attend non-scripture had been repeatedly put into scripture classes at the start of every year and parents were told to provide new notes, in breach of the department's policy.

Students at a NSW public school have been told they need an interview with a deputy principal before they can attend non-scripture.

"Updated permission is required each year for your child to access this arrangement [non-scripture]," states a form attached to the principal's letter, dated February 1, 2018.