Teachers have warned that a lack of supply staff is causing "serious concern" and say the issue is having a negative effect on pupils.
The EIS union says members are reporting "exacerbating stress levels among teachers and managers" and the union has warned that the problem is "impacting on attainment and exam preparation."
Hugh Donnelly, Glasgow local association secretary for the EIS, Scotland's largest teaching union, said: "Efforts are being made to improve the availability of supply teachers but there remain significant problems in primaries and certain secondary subjects.
"Significant staff time and effort is being spent on trying to secure supply teachers and rearranging timetables in order that pupils are provided with cover.
"There is little doubt that there is a link between short-term supply problems and pay and conditions, and the impact of the reduced rates for short-term supply teaching."
The union says the supply situation is the worst it has seen for a number of years and is calling for local and national action to resolve the problem.
A spokesman for Glasgow City Council said: "We share the concerns of the EIS about the shortage of supply teachers.
"This is a national issue and Glasgow has been affected equally with other local authorities."
A Scottish Government spokesman said: "The employment of teachers, including supply teachers, is a matter for local authorities.
"The numbers of teachers trained each year includes an allowance to ensure there is a pool of supply teachers from which schools can draw as necessary."