Maypole1 - in many subject areas, schools would be overjoyed to have to choose between candidates who can be differentiated only by their first or second class degree in the right subject, but who are otherwise excellent teachers.
The reality is that, in some schools, GCSE Physics (or Chemistry) is taught by biologists who may not even have a GCSE in the subject themselves, because physics/ chemistry teachers can't be recruited.
In many schools, even secondary schools, HL Teaching Assistants are timetabled to teach classes. Not to support, to teach whole classes. HLTAs might well not have a degree in anything at all, and they're not qualified teachers- however, they're paid a lot less.
If you genuinely believe that the biggest problem in schools is that there are too many teachers with second class degrees teaching, then you really have no idea.