A friend of mine is a principal at an urban high school. I was talking with a parent at the school once and she said, sort of amazed, “He fires people!”
Yes, it is rare for principals to fire teachers. No, it is not impossible. The process to remove an incompetent school employee may be complicated, but tenure does not mean the same thing in K-12 than it does in universities, and it certainly does not guarantee a job for life. It generally lays out strictures for the dismissal process, which varies by jurisdiction.
As the conversation over teacher tenure heats up, it is important for journalists to provide clarity on the issue: what tenure means, what dismissal requires, why administrators do or, more often, do not follow through on the procedure, and so on. Also, could the tenure rules be modified rather than tenure thrown out altogether? J.K. Wall of the Indianapolis Business Journal did a good job explaining the issue in Indiana; I would love to see more.