Students will learn when they have parents that support learning. Students will exercise, eat right, and do the right things when they have parents that support it. There are exceptions of course, some students don't have the mental capacity, learning disabilities, etc.
Teachers cannot be blamed entirely for students that do not learn. Its more up to the parents to foster and nuture that learning environment. I rasied three children, I've seen it hundreds of times. The students that had parents that cared about education, encouraged reading, homework, quiet place to study,etc. did better in school than the kids who parents didn't. Students with the same teachers. The students whose parents valued education did better then the students whose parents didn't.
Plus, it is good that kids have to experience a few crappy teachers...they have to learn to deal with that adversity, and crappy bosses when they get older. As a parent, you can help them learn to work through it.
The trouble with tenure is that there are some who abuse it. That is true in any organization. In any organization, you have people that abuse the system.