It's bad for education.

School reformers generally agree that the most important education resource is the teacher. But one of the biggest obstacles to putting a good instructor in every classroom is a tenure system that forces principals to hire and retain teachers based on seniority instead of performance.

Read it all and realize it will never change in Public schools. And that's why school choice is so important. In schools where teachers are burnt out and mailing it in, it the kids who suffer. Tenure makes it near impossible to get rid of those teachers. That's why getting rid of tenure would be a great thing for kids and education. But like I said, it won't happen.