Tenure has long protected the academic freedom of teachers and university professors , allowing them to teach freely and to allow for a free and open exchange of ideas in their classrooms. This is the most, in my opinion, important reason why tenure both first existed and subsequently, continues to exist today.
However, while tenure remains incredibly difficult to attain at the college or university level (as it should be) it has become incredibly EASY to secure in most public school districts in the United States today. In many places, teachers are granted tenure after only two years of teaching, which seems to many (certainly to me!) to be far too little time to confer "master" status on an educator. Because tenure has in many places and contexts become so easy to receive, it ends up, sometimes, protecting weak and/or ineffective and/or lazy individuals who would otherwise be counseled out of the profession. For this reason, I agree with the previous respondent who said that tenure should be rethought, certainly at the pre-college level and perhaps at the college and university level, too.
There are good arguments on both sides of this issue.
On the pro side, teachers should get tenure so they will not be in danger of losing their jobs just because some principal comes in who doesn't like them. They also need to be free to teach without having to worry that too many parents will complain about them in some way and they'll be fired for that.
On the other hand, people in all sorts of jobs live without tenure. They can be fired at the whim of their bosses. What makes teachers any different? In addition, tenure makes it very hard to remove teachers who are not doing a good job. Thus, it can be said to hurt students.
I think tenure should be modified: if they teach well, they will not be fired. If not, then they will be more likely to be fired.