In any other job where a contractor is required, their day rate is massive compared to permanent staff. This is because they have to pay their own tax, get no holiday pay or any other benefits and are taking the risk of not finding their next contract particularly quickly.
So, why is supply teaching paid at less than the normal teaching rate (at least in the area in which I live)? It doesn't make any sense at all. Because supply teaching doesn't include pay for any of the school holidays, annually, it works out at minimum wage (give or take).
AIBU to think that supply teachers pay should be at least 50% higher?