This question was asked by Richard Ehisen of the Sacramento News and Review on May 17, 2001. (See http://www.newsreview.com/sacramento/Content?oid=oid%3A6202.)
The answer is: their friends protect them.
On the other hand, these same teachers who protect incompetent peers often attack competent teachers. The reason? Jealousy, competition, jockeying for control of the school.
Wait a minute. Aren't institutions run by women, as many schools are, more gentle, kind, and caring?
Madeleine Albright, former Secretary of State, said, "I'm not a person who thinks the world would be entirely different if it was run by women. If you think that, you've forgotten what high school was like."