In ninth grade, I used OphCrack to find out the admin password, "tyl3r" (lolwut). I never used it for anything malicious. DeepFreeze had a different password, so that was a no go. It was in my engineering class, however, and we used Autodesk Inventor. To import a texture, you needed admin privileges so it could change the default image editor to MSPaint. As such, nobody could put textures in their projects except for me. I had several people ask me to put textures in their projects for them.
I think the teacher knew that I had the admin password, but didn't really care because he knew I wouldn't do anything with it.
We also had a "studentshare" folder, which was a network folder that was shared across all the computers in the classroom. That provided some fun times. But, when you go into a file's properties, you can see who made it (and my teacher did bust a few people for putting proxies in there), so when I wanted to put something questionable in there I used the admin account, which shows up as a blank name in the properties.
Fast-forward to eleventh grade of last year. The admin password was changed, so I tried to boot OphCrack again to find it. The BIOS was locked and disabled booting from USB devices. So my friend and I were messing around, trying to guess the BIOS password, when I had a silly idea and decided to try it. I typed "tyl3r", and lo and IGNORE, that was it. This is interesting because this was at a different school entirely (although in the same district). Guess we had the same lame IT guy set up the computers.
So, I partitioned the hard drives and stuck Ubuntu on my and my friend's computer, and set up GRUB to not show the boot selection screen. You'd have to hold a key to get it to boot in to Linux, and we had our own personal passwords on there as well.
Anyway, fun times. I wonder what senior year shall bring us.