OK, I’ll try yet again.
Schools are under pressure to reduce costs, staff make up 80% + of a school’s budget. In order to attempt to reduce this spending a new role has been created, Cover Supervisor. These staff are employed on a pro rata basis to be the first available cover on site. They are not qualified teachers, they are not paid to teach, they are paid to supervise the class. If a member of staff is absent due to short term illness, these will invariably be the first port of call. If a school is understaffed in terms of cover supervisors they may have to employ supply teachers to cover, inevitably at a higher rate.
If teachers are absent for a longer period this may result in the employment of a teacher on a temporary contract, perhaps to cover maternity leave, this is very common as over half of teachers are women. If a suitable temporary teacher cannot be sourced from the existing local labour market, broadly doubtful, it may be necessary to contact an agency in the UK. An alternative may be to employ supply staff but this is on a zero hours basis and this flexibility cuts both ways.
Furthermore, teaching staff might leave mid way through the year for a multitude of reasons. It is almost impossible to replace teaching staff during the school year, contracts require half a term as notice. This is not the role of a cover supervisor, this requires a teacher. In the short run, this could lead to the employment of agency staff or a supply teacher. However, such supply staff can simply take a week off whenever they like, they are on a zero hours contract. This is probably not ideal.
Sometimes teaching staff may be on site but unavailable to deliver their lessons. The public exam period is a classic example, languages staff have to facilitate speaking tests, science staff have to facilitate practical exams, art and technology staff the same, often for a 2 or 3 day period. The school then has a decision, if teachers are unavailable for a few days do they utilise cover supervisors or supply staff? Taxpayers would probably prefer cover supervisors, parents probably supply staff.
Furthermore, teaching staff are also members of the wider population. They may have long term medical issues. Their families may have medical issues. Teaching staff, tragically, die on the job, sometimes suddenly. Their classes need teaching, this may require the employment of supply teachers until a long term, permanent replacement can be sourced. This is not easy.
Do some teachers swing the lead? Inevitably. Are all supply costs related to this swinging of the lead? No.