True closed-circuit security systems are pricey often unobtainable for K-12 educational institutions. However, there are many times and places where video surveillance would be both useful and appropriate. Front entrances, main offices, final exams, and standardized tests could all benefit. Enter Motion, an open source bit of software designed to turn webcams into motion sensitive video recorders.
While featured on a fairly random blog by a fairly random 15-year old, this post, is quite a fine walkthrough of how to set up a web-based security system using Ubuntu and Motion.
I say it's free because the software is free (including the OS on which it runs) and most of us have a spare computer and/or webcam lying around. Give it a shot and let me know if it works as smoothly as it sounds.
By the way, I'd love to believe the kid who wrote the walkthrough is the product of some brilliant bit of public technology education, but I'm a bit too cynical for that. Check out his blog, though - this particular post is up to 609 Diggs as of this morning.