it only works if your school hasn't gotten as smart as ours and figured out a way to block /all/ proxies. Also it doesn't really work well for interactive sites like twitter or tumblr, but you can get on stuff like buzzfeed.
It's not theoretical. My lab does it, and it's SUPER annoying. Though I'm likely to get promoted, and may be able to take the blocking software off if I play my cards right.
My school allows all that stuff, it's just my mom's school computers I cant do anything on
The REAL question is, how have you never seen this before now?