Unions fear that the move is a deliberate ploy to break supply teachers' service and deny them contractual employment rights, as the council appears to be targeting supply staff who started work at the beginning of the 2009-10 session.
A letter circulated to headteachers last week by the council's education department sparked fears that the staffing review was an instruction.
But the council has since assured teaching unions that the letter should be seen as a suggestion, despite its wording: "This is a measure that is necessary to reduce costs in the future for education."
Nevertheless, some 160 of the authority's 270 temporary teachers in its 29 secondary schools are expected to be laid off this week as part of the council's efficiency drive to cut costs.
A council spokeswoman said: "This is a practice that is widely used across the country and was discussed with our secondary headteachers last month."
She said the move reflected staffing needs at a time when pupils in S4-6 were on exam leave. It also anticipated the position after the exams in June when the vast majority of secondary schools would begin their new timetable and the need for temporary staff would be reduced.
But Hugh Donnelly, Glasgow local area secretary for the Educational Institute of Scotland, said that, although it was common practice in secondary schools to relax cover arrangements for occasional and short- term supply cover during the exam period, schools also needed the time for development work and to prepare for the introduction of new timetables in May and June.
The council spokeswoman added that it was advertising for permanent teaching posts in The TESS today in a bid to regularise the employment status of teachers without permanent contracts.