I got D's in my chemistry class in high school. My teacher said i was pretty much an idiot I couldn't do basic unit conversions. Even though I had a strong math background.
Well I have just finished up my Masters in Chemistry, and am going to dental school in the fall. So anything is possible. High School doesnt matter, as long as you can learn from your mistakes, you'll be fine.
If you tried in your highschool chemistry class, and still did bad, your teacher might have just been a hard ass, but I found those teachers the best, because when I got to college, and took chemistry I rocked it, because he prepared me well. just dont let someone tell you that you cant do it, you can always suprise them.