20-Year Prospect
Barring some kind of Children of Men situation, people aren't going to stop having kids. That's good news for everyone from video-game makers to hungry witches. It's good news for everyone, really, because without kids, there'd be no future population.
Kids also need to be educated. Sorry, guys. That's why we cram you into schools. We don't need an entire population of Homer Simpsons, thanks. Education means schools, and schools mean principals.
So yeah, school principals aren't going anywhere. Even with automation, the bane of pretty much every other job under the sun. This position is probably going to remain in the province of humans, or at the most, cyborgs.
The statistics for job growth, though, are all over the map, ranging from slightly worse-than-average (source) to about average (source). This looks to be mostly because certain areas' populations are growing (the west and south), which means more schools, while others (north and east) are staying comparatively stagnant (source).
The job will change as technology does, both for educational purposes and the ways in which students get into trouble. The job itself likely won't be too different, but you'll have to keep up on the latest devices your students have on hand. You don't want to be the one shouting in confusion, "They have ray guns now?!"