Why do schools use supply teachers?
In short
Cover is not a single problem with a single cause. Schools use supply for planned reasons such as training and maternity leave, and for unplanned ones such as sickness. Vacancies and recruitment delays create a third, longer-running kind of need.The main reasons
- Sickness absence. The largest single driver of day-to-day cover. In 2024/25, 61.8% of teachers took some sickness absence, averaging 8.1 days each.
- Maternity, paternity and parental leave. Planned, and usually long-term.
- Vacancies. A post that is advertised but unfilled still has to be taught.
- Recruitment delays. Teaching posts run on long notice periods, so a resignation can leave a gap of a term or more even when recruitment succeeds.
- Training and professional development. Releasing staff for CPD, moderation or leadership work requires their classes to be covered.
- Long-term specialist gaps. Hardest in subjects where recruitment is weakest, and where a non-specialist cannot easily fill in.
Why this matters for reading the data
Because the reasons differ, the measures differ too. Absence, vacancies, temporarily filled posts and supply expenditure each capture part of the picture and none captures all of it. A school with high supply spending might have high absence, a hard-to-fill vacancy, a generous CPD programme, or a staffing model that plans for cover.
Association is not cause. Schools with high supply spending may also show high absence or vacancies. That is a pattern worth exploring, not an explanation.