The natural other question is of course how do we get bad teachers out of the classroom? The next question is how do you tell who is a good or a bad teacher. (Did you ever notice that the important issues lead from one question to another and then another and on and on?)
The situation is that we have great teachers, bad teachers and every kind in between and they all pretty much get paid the same amount of money. Why would you work hard, learn new things and bring innovation into your classroom when the teacher who has his students read magazines all class long is going to get paid as well as you? Where is the incentive to improve? How do you keep your energy level high when you know that others are not working much at all.
If you are a principal and you have teachers who hardly know their subject or who are poor communicators how do you get rid of them once they have tenure? How do you motivate good teachers to try new things and improve? Oh sure you can make teachers attend great workshops but that is not the same as getting them to use what they learn. What is the "hammer" or "carrot" you can use?
Tenure is only part of the problem of course. Unions are another problem. Both can be serious obstacles to removing teachers who are not performing. (Admittedly they can also be highly valuable in protecting good teachers from school politics.)
Of course even if you can get rid of a poorly performing teachers how can you recruit good ones? Money is seldom an option. Job satisfaction is one drawing card. The reason that senior and high performing teachers so often wind up at schools in affluent areas is not money. It's also not race. (There are economically depressed areas of all races and they all have a hard time attracting great teachers.) The issue is that students in affluent areas are often better behaved and more motivated than those in economically depressed areas. It's cultural not racial although there are racial/ethnic groups that are highly interested in education and schools in those areas attract great teachers regardless of economic status. A number of magnet schools in poor, under represented minority areas attract great teachers based on an outstanding school culture.
Building a great school culture involves something of a chicken and eggs problem though. One needs great teachers to create a great culture but you cannot often attract those great teachers without a great culture. The reason some charter schools are able to succeed is that they attract teachers who want to help build a culture from scratch. Closing all the existing schools and replacing them with new schools from the ground (in terms of personnel) up is hardly practical though. If one could put great principals, people with vision and the ability to replace teachers, one could (in theory at least) start to replace teachers and rebuild a culture. This would of course involve getting some buy-in from the community. Unless parents support the cultural and behavioral ideas of a school it will likely fail.
There is no easy answer here. Money is one way to attract great (knowledgeable, energetic, good communicators) teachers but that is hard to come by. Getting rid of teachers is difficult because of unions, contracts and the very real need to protect people from arbitrary and capricious principals (who do exist). The focus on public school reform seems to have gotten down in short term solutions that revolve around teaching to specific high stakes tests. The focus around identifying good teachers is focused on paper work and tests that often have little real connection to classroom results.
Changing the rules is difficult. This explains why charter schools and often private schools are able to create and maintain high standards of teaching. Is there a willingness to make big changes in public schools that allows large scale replacement of teachers and rebuilding of culture? Somehow I think not. I wish it were otherwise.