State school principals must offer special religious instruction when accredited instructors indicate they are available, the latest ministerial directive says.

The state government has clarified principals' responsibilities in special religious instruction, with an Education Department memo saying the sessions are ''clearly opt-in'' for parents.

The directive said principals must offer parents the ''opportunity'' for children to be given religious instruction if the principal received notification that an accredited instructor was available.

Under the directive, principals must also request a copy of the instructor's formal accreditation.

Teachers must supervise the religious instruction of students. But students who opt out must be supervised by another teacher.

The directive said once principals offered religious instruction, they could withdraw it if there were ''insufficient resources'', including teachers or classrooms.

Fairness in Religions in School campaign member Scott Hedges said this requirement meant principals could cancel religious instruction for a lack of resources.

''It gives schools a clear pathway to terminate this program,'' he said.

Victorian Principals Association president Gabrielle Leigh welcomed the ''clarity'' about principals' obligations.

But Australian Principals Federation president Chris Cotching said the directive placed an unreasonable burden on principals to check the material instructors used in the sessions.

Principals would have to ensure students who opted out were not taught from areas within the regular curriculum, but they would have to be ''engaged in educationally valuable activities''.

Australian Education Union Victorian branch president Meredith Peace said state schools were secular and should remain so. ''We're opposed to these programs being run within school time,'' she said.

Ms Peace said there was room within the existing curriculum for education about religions, but it should be taught by qualified teachers.

She said many primary schools would struggle to provide a teacher to supervise the religious instruction and another for those whose parents had opted out.

The main provider of Christian instruction in Victoria, Access Ministries, said the parents of 90,000 children already choose the ''educationally sound program''.

The directive also said instructors must not provide any rewards of a ''tangible nature'' or inducements to convert students.

The religious instruction sessions must be no longer than 30 minutes a week.