I guess the other thing that exacerbates the problem of children's grammar and spellings is the fact that they no longer read (in general) as much as children used to, or were expected to by their teachers.
Even if children/young people read articles on the Internet, they are not necessarily correct in their spelling or punctuation or even syntax for that matter. The birth of the world wide wonder net has also given rise to the de-standardisation of English. Reading for content, not for accuracy.
It is a sad day to me.
However, I did also hear on Radio 4 several months ago that spelling (not punctuation as that does follow rules) is a skill you either are born with or not. Spelling does not equate to teaching or ability, but rather to whether you can or can't. I am a poor speller in my opinion, but I try to look words up if I'm not sure of their spellings. However, there really isn't any excuse for using the incorrect word or punctuating poorly.
It all comes down to sloppy speech (as has already been said) and a move away from reading.
I think it is true that the Sun Newspaper has an average reading age of 7 meaning that on average, there won't be any words in the Sun which a 7 year old couldn't read. Says it all for our Nation really, doesn't it.