chpas -
Je ne sais pas 
in the warbled, 4-words-in-one-syllable speak of
les ados
ficher (v) -
1) in the sense of
Qu'est-ce que tu fiches ici?, What are you doing here?
2)
Je m'en fiche! (s'en ficher) - I don't care, though probably closer to I don't give a shit!
3) A million other uses here and there.
la fac (nf) - short for
la facult, uni. That's used a lot and gets ya browny points - good when talking about yourself and your future. So does, though probably obviously, 'le bac' for 'baccalaurat', HSC.
hyper-+adjective - 'hyper' gives more emphasis to an adjective, like hyper-cool!, hyper-chaud. Though personally I haven't seen it much...
Vachement - also good as a substitute for 'trs', which we use a looot, almost too much.
About using slang though. There's always a line about when something comes too slangy that the examiner doesn't like it or just doesn't understand. In an exam situation especially with emails it depends sooo much on how much the examiner/marker knows, and if the examiner/marker is 100 years old who still speaks the slang of Charles de Gaulle we could be up the creek if we used to much. I used
a craint! for 'that sucks!' in a prelim exam last year and the teacher circled it because she had no idea what it meant, which I'd picked up of some subtitle on a DVD or something on the net. I can't remember... So it's good to verify how often it's used and how widely it's used before we'd use it! Hehe. It's really tricky.
http://french.about.com/library/vocab/argot/bl-argot.htm is a good site that has an alphabetical run down of some slang. Might be worth checking out.
Anyway, just some random thoughts comme d'hab(itude - another one!).
Chpas
