I can't blame everything on my teachers but I can't remember anything my English teacher said to me that helped me improve my work.
Focused feedback is proven to be the most effective way of improving a students' grade.
My drama teacher at high school once said that my 'comedy hobby horse' riding for a Spoof Western looked like a 'drunk tramp trying to dance with a broom'. With that comparison, I understood clearly what she meant and modified my acting accordingly and got an A eventually.
Share here what the most effective thing an English teacher has said to you to improve your work. You never know - you could change someone's grade.