Tenure isn't some magical invulnerability to being fired. It simply slows the process to a crawl and forces the collection of a mountain of evidence to fire someone for cause.
If someone intentionally, and repeatedly, violates the curriculum you can fire a tenured teacher.
You can't acquire that mountain of evidence in the 12 months between getting a ban on evolution passed and having it repealed in federal court.
Tenure doesn't give them immunity to the school board and/or other administrators.
It does last long enough that abuses of the system and/or unofficial pressure to be resolved before the teacher actually loses their job.
Firing a teacher with tenure can take 12+ months.
Clearer?
Which is it? Can the school board fire teachers for not following the curriculum? If so, then school boards can stop teachers from teaching evolution. If not, then teachers can choose to teach Creationism. Either way, tenure doesn't matter.
Another way to phrase that would be: Teachers can violate such a ban temporarily.
It isn't blanket immunity and that isn't what I said.