I'm divided. In high school i learned nothing in any of my English classes. The last thing I learned in public school English was the standard five-paragraph essay in eighth grade. While many will whine about this, it's also the single most useful thing I've ever learned about writing. It is a VERY hand, VERY clear, and VERY professional method of argumentative writing.
That said, I had a creative writing teacher in 12th grade (the course was also available to freshmen on up) who was the single most supportive and helpful mentor for writing aspirations I've ever met. She taught me a great deal about clarity, character, creativity (and other 'c' words) as well as finding inspiration. She will not be forgotten. My little brother and sister also took her class and will testify to her quality.
In addition, I also had a World Literature elective class taught by a little Jewish woman we referred to lovingly as "the hyperactive chipmunk." She opened my mind to reading analytically without being tedious or slavish... to ENJOY reading analytically and get useful information from it. While this didn't shape much of my writing style or ability, it does drive a lot of what I write about.