Look I'm sure we have all had the experience of being taught by very good teachers and conversely by some truly appalling ones. Furthermore we have to accept that there are huge variations in teaching standards and qualifications, some are indeed Oxbridge educated as several at my school were and therefore would tend to go against the title of this thread. However, there does seem to be many instances of under-qualified people making it into the profession, for example it is my belief that someone who has less than an A grade at A-Level in the subject they wish to teach in is simply ill-prepared to teach a class to that standard. That this occurs and also that teachers are often forced to cover in subjects they have no specialist knowledge of often leads to a poorer perception of the teaching profession. The notion that if people were able to excel in their subject they wouldn't teach is truer in vocational subjects I think such as IT and Design Technology. This is because there is an obvious industry where these degrees could be utilised whereas for a geography degree for example teaching is more of a logical step. If we want to raise the standard of teaching we have to increase pay to attract the best calibre of people, at present the best aren't being attracted but I think the situation is a long way from being one in which the worst students end up in teaching as was being implied by the OP.