I left school because of mental health problems and so didn't even get to sit my highers. In addition, I allowed students (and most worryingly, teachers) to convince me that I wasn't smart enough for science and should do 'something more practical'. I washed cars, became a travel agent and eventually went to night classes 4 years later when I had recovered, where I did human biology/chemistry (got two As and felt good to give them all the finger). That got me into a college doing biomedical sciences (with a guaranteed place in third year at my current university), at which I got top results all the way through and an A in my graded unit both years. Now I'm in third year and very excited for my future in academia. I'm also doing higher english this year as a distance learning course because writing is such a vital skill.
Do not give up. Ever.