Use Portable Apps (Score 1)

Have a clean install on your laptop and then use portable apps http://portableapps.com/ for anything personal.
Use encrypted filesystems for personal data.

No matter how you look at it; a laptop is portable and can easily be stolen. The question is not whether you'll let others use it from time to time, but how you personal stuff is protected in case it gets 'misplaced'. If you are prepared for theft, making that friendly gesture to a co-student won't bothr you a bit. At least security-wise.