The traditional justification is that teachers need to feel free to discuss ideas that social trends or local political whim may disagree with. They may even need to take a Devil's Advocate during a lesson and argue a pretty unpopular position that they don't personally agree and which would sound awfully impolitic out of context.
E.g. To protect them from being fired for reading from a given book. Or teaching evolution. Or teaching sex education. Or covering a topic like communism or socialism in any way other than demonization.
By that same logic, though, teachers are then protected from being fired for reading from the Bible, or teaching creationism, or teaching abstinence-only birth control, or covering a topic like vaccination or astrophysics in any way other than demonization.
Tenure isn't the appropriate approach here. Enforcing Constitutional law - like separation of church and state - is far more useful to actually achieve the desired results. If teachers are being fired for teaching actual fact, then they need to be bringing that to the legal system and turn it into a Supreme Court case. This whole "it protects secular teachers" argument is a silly justification for the negative impact California's tenure-fetish has had on students and younger teachers.
There's no constitutional concern here. Tenure would provide no protection for a teacher who was using their position to preach. Any talk of creationism or reading from a holy text would have to be part of a lesson that didn't run afoul of the establishment clause.
Neither does tenure necessarily provide protection for a teacher who refused to teach the curriculum to such an extent that they failed their students. Tenure is about protecting teachers from being fired for controversial positions, not protecting them from having to teach the material they agreed to teach.
I can't speak to the particulars of California's tenure or contract in general. Maybe their contract really could protect a physics teacher who taught epicycles instead of heliocentricity despite failing class after failing class.
I think you're mixing up my points. The Constitutional concern doesn't lie with tenure itself; enforcement of the Constitution would replace this rather far-fetched excuse for maintaining tenure as some holy trait. The legal system already exists to help teachers in cases where they're punished for teaching science instead of religion in government-funded institutions, since doing so already runs afoul of the establishment clause; a tenure system doesn't really add much, and (in my own observations at least) only seems to benefit veteran teachers at the expense of students and newer teachers.
E.g. To protect them from being fired for reading from a given book. Or teaching evolution. Or teaching sex education. Or covering a topic like communism or socialism in any way other than demonization.