Brycie, I'm not saying that underperforming teachers should not be dealt with, nor removed from the classroom.
There are teachers out there who are not up to standard. Some are very inexperienced, and get much better very rapidly over a relatively short period of time. Some teachers have huge changes in circumstances which mean they can't cope for a period of time with their current workload. Some teachers (thankfully a tiny minority or none in most schools) are just plain crap.
As I said, some headteachers are not the greatest at identifying the best strategies for dealing with underperforming teachers. Inexperienced teachers get much better with the right support, training and mentoring. Teachers struggling with personal circumstances may be helped by going part time or relinquishing responsibility posts. Crap teachers need to be out of the profession if they don't show the capability to improve.
Too often, headteachers use tactics when trying to 'manage' underperforming teachers that leave them wide open to union intervention or even successful tribunal claims.
Either way, it would be very rare for a school to choose to, or even be able to, replace a poor or off-sick-with-stress teacher with anything other than a short-term supply teacher or cover supervisor.
Cover supervisors - employed by many schools - are often not qualified teachers. They merely 'supervise' students completing work set by someone else. Short-term supply teachers don't mark work, carry out assessments, write reports or attend parents' evenings. Long-term supply would only be used if the long-term nature of the absence had already been determined.
Either way, it is not a great state of affairs for the students, but it is up to the SMT to ensure that this is dealt with in a way that doesn't leave them wide open to a successful claim for constructive unfair dismissal.
What some headteachers are guilty of - and the blame must be laid entirely at their door - is 'recycling' poor teachers. Rather than following procedures and dismissing inadequate teachers, they 'persuade them to find employment elsewhere' - often in another school - with an agreed acceptable reference. How does this help the situation?