I'm not arguing that teachers shouldn't be held accountable for their actions - in fact I would argue that more than any other profession teachers are held accountable for what they do not only in the classroom but outside of the classroom as well. Try teaching in a rural community - you'll soon find that as 'the teacher' you are scrutinized no matter where you are or what you are doing. What I am arguing is that our definitions of 'good' or 'bad' are very subjective and don't take into account the fact that different teachers are going to appeal to different students. If you actually advocate the mass firing of 10 % of workers in every profession (not a practical solution, in my view), wouldn't you want to have a consistent definition of what they would need to be doing to be considered a 'good' worker? You haven't offered any suggestion of how you would measure success in the teaching profession - just the claim that it can be done. The problem with teaching as a profession is that there are expectations made of teachers (for instance, socializing students or solving major behavior problems) that have nothing to do with their stated job description of disseminating information.

Your viewpoint also neglects the fact that both hiring and firing in schools are generally politically motivated - and by this I mean the internal petty politics of any bureaucracy. Teaching is incredibly nepotistic, and it is difficult to get a job in urban areas in the Canadian context without being personally known to the person doing the hiring. While school districts have a legal obligation to post jobs, very often the postings are a formality as administrators (not surprisingly) want to hire people whom they are already familiar with.

Furthermore, I often wonder if the teachers that are being complained about as being 'bad' are simply working outside of their expertise. There are some positions in junior high and high school that are notoriously difficult to fill - notably math, science, and French courses, and are very often filled by people who lack the pedagogy relating to those topics. The vast majority of people who choose to go into the teaching profession come from a English/History background, so most of those positions tend to be filled by tenured teachers who get first pick of their courses, and the French/math/science slots get filled by first time teachers who are willing to take any position that they can get. This problem is compounded in rural areas that struggle to attract anyone at all, and then you end up with new teachers teaching multi-age classrooms, and teaching EVERY subject for up to 4 or 5 different grades. After all, the one and two room school is still very much a reality in rural areas.

I will concede that my suggestion of allowing students to choose their own teachers suffers from being urban-centric, and would be impossible to implement in rural regions.