Personally, if those kids want to use that sort of stunted
language, it's their choice. Ultimately when they get poor grades
for English they only have themselves to blame. When the usage of
such stunted grammar becomes a habit, it's extremely difficult to
reverse it.
My tuition student, for instance, keeps putting in SMS lingo in his
sentences. 'Forgot' becomes '4got', 'tomorrow' becomes 'tmr', and
so on and so forth. He protested strenuously (to no avail) when I
penalised him heavily for using such terms. Heck, his eyes pretty
much popped out of their sockets when I told him to start using
proper sentences in his SMS and MSN conversations.
What really gets on my nerves is when such people mock and poke fun
of others who take the effort to speak standard English. For
instance, my neighbour used to chastise me for using proper English
in our MSN conversations.
She said that she couldn't stand the fact that I capitalised the
first letter of every sentence and used proper spelling like
'tonight' instead of '2nite'. 
Of course, she started to sing a different tune altogether when she
got C6 for her GP in her A Levels.
The bottom line is, if you want to use your own slang, then that's
your choice. However, if you're going to let it creep into your
schoolwork, then you've only got yourself to blame if you do badly
for your English.
In addition, if you're going to mock others for using proper
English, then I say you're self-delusional for thinking that poor
English equates coolness.
