go to any state records, and you can find teachers are non-renewed every year! In Florida, it averages over 2000 a year. If you commit a crime you are gone. Even if you are simply a bad teacher, you can be fired if the administration documents the problems.
"Tenure" (sometimes called continuing contracts) ensures that teachers have due process. In other words, you can't be fired arbitrarily and the teacher gets a fair hearing. There has to be a cause for the firing. Teachers still have to keep up their credentials and are evaluated annually. The vast majority of teachers are pretty good, but the ones who are not get played up by the media.
With college professors, it's similar except tenure also protects professors from being fired for political reasons, so they can include controversial topics in class without fear of retaliation (called academic freedom). Most teachers have a curriculum established by the school board, so academic freedom is not an issue for them.
Of course, the best protection is usually the local union, because without being organized and having a collective bargaining agreement, there is no real enforcement of tenure even if the state law allows it. The union will fight violations of the contract and represent teachers if they are in danger of being fired - and the primary reason for representation is simply fairness.
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