We don't need no stinkin' grammar...

I want my students to learn to write competently. So after months of scoring paragraphs and essay, followed by individual corrections, I have decided to teach grammar. In my last post, I shared that many of my students are failing. The main reason is that they don't turn in the work, but we also need to improve skills. What follows are a few examples, unedited, of student work.

A Sophomore in the College Prep Track

Electronics is being a big issue in school, administration think its not good source for student concentration and mind multiply teachers think its not doing any good for student that its just a distraction and a not a good at all very few teacher think its good. Students are against this issue they think the opposite of this, their opinion is that mp3 players such as I pod's are good them and hllps them stay focus and concentrate on their work or materials to comlete and not only that but they say that their work is right and complete with almost no errors.

A Senior in the non-College Prep Track

...Now the school had to call the cops but what I am trying to say is a lot of incidents happen like this in the school and for someone getting arrested for something so stupid just because he got a little angry is ridicules but what I think the school is trying to do is because the school is so bad and the cops had to come 170 times out of 80 to the school last year they feel that they need someone to help out and there going to start cracking down on kids that actu up and have bad behavior to rty and change the schools weaknesses with an officer.

A Hodge-podge:
"Heavy Metal VS Today's Rap, what I think about the subject, many people today listen to rap and many listen to heavy turtle, a lot of violence today is based off rap and the "gangster" image the is posted everywhere by these rap artists they put a bad image all on the teenagers today."

"Cell Phones for teenagers are their life. They have all diffent kind of information on them like serval texts messages on them that may mean a lot. Tehachers on the other hand are getting over per causes about them."

"I believe hiring younger teachers are more effective, because they are at a younger level and closer in ages with us, and understand our differences."

"Some of the reasons the officer will have on the student body will be the fact that he's already at the school and I know some students will find it as a threat, take it as if he him self is being disrespected and maybe causing more problems when the officer isn't at the school."

Here is where I can admit a limitation of mine. I really don't know where to begin. Will grammar study fix the serious flaws in my students' writing? Probably not. So what do I do to help these students, especially the college prep sophomore?
At some point, don't I have to go back and re-teach what they should have already learned? I've got to believe they've been taught about nouns, pronouns, verbs, adjectives, adverbs, conjunctions, and prepositions (we can leave out the interjection.) I've got to believe they've been taught that every sentence is a unit of thought made up of a subject (always a noun or noun phrase) and a predicate (always includes a verb). I've got to believe they've been taught that a complete and proper sentence makes sense.
But clearly, they haven't learned it. I don't know what methods were used back in grade school and middle school, yet it is evident that the basics need to be taught.