I stumbled across a great explanation of why hard drives on Linux computers rarely need defragmenting. It's a common question from students and, as laid out in this post, makes for a great quick lesson on file systems and hard drive management for high school and college students just beginning their study of computing and computer science.
While the article itself basically hands you a lesson plan (one could even imagine in-class activities with students filling in squares on graph paper, rearranging their seats, or any number of other hands-on demonstrations of bits moving around a simulated hard drive), the essence is that FAT/NTFS require frequent defragmentation because they order files sequentially. Linux file systems, on the other hand, leave room between them on the drive, allowing them to expand, shrink, and move on the fly without being broken into chunks (or fragments) as frequently happens in Windows file systems.
It's a cool read and very understandable for students from a variety of backgrounds. Enjoy!