Local authorities are scaling back study leave, partly because new National 4 qualifications do not have an external exam. That means more pupils will spend time on school premises than before.
However, study leave frees up preparation time for teachers, and, with the imminent introduction of a raft of new qualifications, is viewed as more valuable than ever.
Teachers' union NASUWT Scotland (National Association of Schoolmasters Union of Women Teachers) discovered the trend while carrying out a survey into the impact of the new Curriculum for Excellence.
Union president, Mike Corbett, said: "Stopping pupils from developing an independent approach to revision seems to be a curious approach and does not sit well with what we are aspiring to do for and with them as confident individuals and successful learners.
"The huge amount of development work which secondary teachers manage to do during study leave would be significantly curtailed, a strange thing to want to have happen at a time when we are in the midst of implementing new national qualifications," he said.
Mr Corbett accepted National 4 courses did not demand study leave, but said relatively few students would sit these exclusively.
Hugh Donnelly, secretary of the Glasgow Educational Institute of Scotland union, said traditional study leave allowed a "little bit of breathing space" for teachers to plan ahead - particularly given the pressures they had faced this year with the introduction of new qualifications.
He acknowledged study leave was not contractually guaranteed, but said it was a tradition established over time. Glasgow was phasing it out, he added, with Nationals affected initially, but study leave for Highers also ultimately due for elimination.
He feared schools would struggle to find space for students who would normally be on leave during exams.