Of course some students will do it. But IMO those with poor literacy and numeracy skills probably know about it and don't really care from my perspective. I know I don't have great spelling and grammar but I am constantly trying to improve it. Those who don't recognise that non standard spellings in formal writing is not acceptable definitely need to address it. But as I said, I doubt they care. To some people (stupid I know) an uneducated persona is seen as a desirable attribute. I 100% agree that textspeak is not for formal prose, but I think on laid back forums such as this, social networking sites, social interactions etc., it is perfectly valid.
Not only is it valid, it is absolutely necessary. Groups have their own identification, and this can be reflected through a unique use of language.
This has always got me. My sisters constantly tell me off for putting up an interesting status update, whereas people are applauded for status updates about some crazy weekend. Having said that, the tables will turn when your in your 40's where being educated, rich and respectable are wanted attributes.
This is a good example of such identity/language clash. Where one group is into wild social activities for fun, their speech adjust to reflect this behaviour. Later in life, when their mentality changes and they wish to be identified with people like you, you will also notice a change in their language.
There is nothing wrong with the teens saying what they want to the way they want to. So long as they can understand each other, and so long as they are being themselves, who are we to judge?