My husband took a Persuasive Writing course last semester through an on-line college. All of the students were required to submit posts regarding their goals for the class on a forum similar to this one. Reading the posts was saddening, and the level of writing skill would have embarassed a fifth grader.
The good news is that before the students were expected to begin their actual final papers, they were required to complete several weeks of lessons on grammar, sentence structure and punctuation. The quality of the posts definitely improved following the lessons.
Now this on-line college is all self-paced, to a point; which means that the student must be self-motivated in order to learn what is being taught. (Yes, I still get confused with colons and semi-colons, but I try.)
So, the bottom line is that if you're not learning what you need at high school, take advantage of other options, such as tutoring, or lessons like the ones DG posted. Learning to write correctly is definitely an advantage in life. When you know the rules (grammar, etiquette, whatever) you can choose to ignore them, but at least you'll be a step ahead of most folks.