Union leaders have claimed teaching staff have not been consulted about a reduction in the Highland school week, which will be introduced over an 18-month period from August.
The local authority decided to move towards a 33-period week back in 2014 after extensive discussion about “educational benefits” with a focus on sustaining rural education.
The council intends moving towards a common timetabling format to allow wider access to senior courses.
The first stage of the process is a 33-period week for secondary school pupils with an early finish on Fridays.
Speaking yesterday, a spokeswoman for the GMB union said: “We don’t know the implications for staff, but we would be against any reduction in school hours which would affect staffing levels and children’s attainment.
“There’s been no consultation with staff.”
A spokeswoman for the council responded that any change in hours “would be a local decision, reached by the head teacher in consultation with staff and parents.”
She added that primary schools have a choice whether they finish early on Fridays.
The GMB has also ordered members in clerical school roles to work to rule amid changes to their conditions.
This has arisen, due to the “clustering” of Highland schools involving the sharing of head teachers in remote areas.
The instruction affects a total of 200 support assistants and janitors on annual contracts of about £13,000-£14,000.
The union’s branch organiser Liz Gordon said: “We’ve told all school clericals to please work to your job description because that’s what you are paid to do.
“They consistently work beyond their job description because of the nature of the school environment.
“If the council want them to do more, they can go through the job evaluation scheme and come to a proper level of grade.”
On that issue, the council spokeswoman said: “We’re undertaking a review of support structures with the objective of improving support to schools and head teachers as part of a review.
“We recognise the vital role the school office plays and have formed a working group to develop recommendations.
“The group includes office staff, head teachers and unions.”