For all the rhetoric about the need for teacher accountability, school principals and district superintendents already have the power to fire teachers. Yes, it's true: Primary and secondary school-teachers do have a form a tenure, but it's not the same kind of tenure that university professors have. In primary and secondary education, tenure simply means that there is a more formalized process for firing a teacher. More hoops to jump through, if you will. But, principals can jump through them if they really want to.
The logical question to ask is, so why don't they? They don't fire the bad ones because, simply put, they need butts in seats. Or, rather, feet in shoes in front of the blackboard. If you have 4 teachers for 100 students in 3rd grade, firing one and increasing the average class size to 33 students isn't a tenable option. As a result, the principal must hire a new teacher to replace the one he just fired, and therein lies the rub.
There simply aren't good teachers on the market at the prices that school districts are willing to pay. That qualifier at the end of the sentence is important, so read it again. Many of my cohort from grad school were fantastic teaching assistants in the classroom (many were bad, too, but we'll save that discussion for another time). That's not to say that they would automatically make good primary school teachers, but if I had kids, I would wish that those excellent TAs were in the classroom. I'll bet that many people on HN know similarly talented people from their time in university, who were natural leaders and mentors.
Unfortunately, very few (read: none) of those people who are good at teaching are employed as teachers. Why not? It has nothing to do with teacher's unions or certification requirements or institutional bureaucracy or barriers to innovation, and everything to do with the labor market. Namely, a good engineer or scientist with a masters or doctorate can out-earn a teacher by a factor of at least two. Then there's the fact that an engineer employed as an engineer isn't required to restrict his use of paid vacation days to just a few weeks each year (a couple high-school teachers with whom I'm still in contact tell me that they've never been able to take their kids on vacation for spring break because their spring break didn't line up with their kid's spring break).
So, I think the solution is easy: Pay teachers a salary that is competitive with other high-skilled professions, and you'll get good teachers.