I agree we probably need to look at modifying the tenure system and ideally instituting some sort of accountability measure, but absent other reforms such measures aren't the panacea that many portray it to be. I also have to say that it isn't quite as hard to fire tenured teachers as portrayed in the media, and that teacher tenure, or teacher unions, do not exist everywhere.
Educating children correctly is a complex, demanding task, so it requires a skilled, highly motivated workforce to accomplish.
We've undertaken a fair bit of experimentation in different types of school settings during the past decade and what seems to work, union or no union, are settings where the workforce of the school is highly motivated, talented by a number of measures such as academic or other achievement, spends a lot of time with the students measured in school hours and school days (at least in settings where the student populations are behind the national average academically, which is where the majority of the discussion over school reform is focused), and engages in a program meant to socialize students towards positive interactions with others in a variety of settings.
The problem with such schools is "scaling them up", so that the reforms that make them successful can be applied to schools elsewhere, and the scaling problem always comes down to money. Those schools depend on a workforce comprised of young people recruited out of college through programs like Teach For America where the recruits agree to spend two years teaching before moving on, usually to another career. The turnover at such places is usually high, not because the teachers wish to leave education, but because they want to move on to other careers where they are paid more.
In the end teacher pay isn't the only factor, but it is a huge one.
At some point we have to ask ourselves why we keep attracting a certain type of person to a position.
Teachers, almost alone among the white collar occupations, felt it necessary to unionize in large numbers. Nurses didn't, at least to the extent that teachers did. Neither did engineers. Neither did doctors, lawyers, or accountants.
Teachers unionized in part because the pay often was very low (there was a time when someone just out of high school could obtain a manufacturing job in some areas that paid the same salary as a teaching job that required a four year college degree), and in part because the working conditions were often very bad (as was mentioned by others). There were reasons for those, and the reasons lay mostly in the way teaching profession was viewed historically, as unimportant to an economy that relied mainly on manual labor, and the fact that for a long time teaching profession could count on a labor force that was shut out of most other occupations.
Those conditions existed when teacher unions were formed, but they do not exist now. Both teacher unions, and the society that employs them, need to recognize that fact.