Research by The Herald suggests Scottish schools have as many as 500 vacancies as pupils prepare to return to the classroom.
A total of 20 councils reported 404 unfilled posts evenly split between primary and secondary schools, while the remaining 12 local authorities were advertising for around 100 jobs, the newspaper found.
The Educational Institute of Scotland (EIS) said the figures show the need to step up efforts to attract more qualified teachers, but it warned standards must not drop amid controversial proposals for fast-track pathways into teaching.
EIS general secretary Larry Flanagan said: “Vacancies at the start of term are clearly a cause for concern.
“The situation highlights the need for a concerted effort to attract a greater number of qualified teachers into Scotland’s classrooms.
“If we are to ensure equity of provision across the country and address the poverty-related attainment gap, then it is essential that our schools are fully staffed.
“As the EIS has repeatedly highlighted, making teaching an attractive career option will require greater action to reduce excessive teacher workload and a significant improvement in teachers’ pay and conditions.
“However, we must also ensure that our high standards are maintained by continuing to guarantee that only fully qualified GTCS-registered teachers are teaching in our schools.”
A spokesman for council body Cosla said: “The issue of recruiting teachers has been problematic for a number of years and sadly remains so. There is no magic wand that can be waved as our young people return to their classrooms over the coming days.
“This is a long-term problem that will require a long-term solution - especially for those councils who have real issues with recruitment. Councils do their best to attract teaching talent to their area with many offering incentives.”
A Scottish Government spokesman said: “Although teacher recruitment is a matter for local authorities, we recognise some areas have faced challenges filling vacancies, which is why we have taken decisive action to recruit and retain teachers.
“This includes investing £88 million this year alone so every school has access to the right number of teachers. This investment has enabled local authorities to maintain the pupil teacher ratio and halted a period of steady decline in teacher recruitment by councils, resulting in 253 more teachers last year - the first substantial increase since 2007.”