Well, at least at the school I'm at (a pretty average suburban academy outside Glasgow, Scotland) we're not taught grammar rules. We get a basic-ish grammar primer in primary school, but that's about it. We're not even taught how to use 'whom'. I used to hang around a fan fiction writers' discussion board a few years ago, and much grammar checking took place there, most of which went straight over my head due to the above lack of grammatical education. The same goes for when I run a grammar checker and read what I've supposedly done wrong. And I probably have the best grammar and spelling in my school...
On the Internet, things are even stranger. I hate contractions like 'r u ok?' and 'c u l8r' (probably from their associations with AOL'ers and script kiddies - although to my horror they're becoming mainstream now with their use in text messages...) but when I'm speaking or typing in real-time - for instance in IRC - my grammar gets, for lack of a better word to describe it, 'weird'. I think my brain runs faster than my fingers and/or mouth at times, which results in strange grammatical constructions and words coming out in the wrong order - verbal output-only dyslexia?
And don't forget that the Net has its own language, composed of cast-offs and renegade phrases from all over the world. Read the Jargon File for some good ones, or lurk a while in any message board or, even better, IRC channel. Grammar rules are loosened. After all, people'll understand what I say if I say 'better-ish' or 'paintableness factor', or if I talk in one huge run-on sentence. I could even talk in l33t if I was desperate.