I have to strongly disagree with that. We've hosted six exchange students over the years, and most of them are very strong students. They generally take the most rigorous academic classes (AP and honors) and excel in them while becoming fluent in the everyday use of English (they study it for several years before coming over, but that's different from using it all day, every day for 10 months). The last two were here in the last three years, as texting was becoming more and more popular, and both use that in their emails to us, as well as instant messaging. They certainly know the difference between that and writing a paper for a class.
I don't see it as any different than the many acronyms we use here at DC - it just saves time when IM'ing and character usage when texting. No biggie