First off, I would like to say respond to another answer. To "Sane," if you are going to criticize other people for bad grammar, could you please check your own English skills? Starting a sentence with "Agreed," replacing "it's" with "its," and misspelling "sensible" are obvious grammar mistakes. Oh, and to attribute children's poor grammar to "liberalism" is a stretch, and I think it is merely you letting your political views intrude on education. In addition, I have never had a course called Pagan Rites or Lifestyles of a Homosexual Family. By the way, there are homosexual families in America? Since when could they marry and start a family? George Bush would disapprove.
To answer the question, though, I would have to say that it is not only deficient skills that cause poor grammar, but also laziness among today's youth. Many, though not all, children do not want to take the time to use proper grammar when they are not being graded for it.