Kind of depends what class this is
If it's a Maths or Science subject, this might have something to do with the content you're covering and how you revise. If it's something like English or Humanities, it could be the structure of your essays and work. What kind of feedback are you getting from the teacher? Could you ask specifically what you could do to get your grade into the 80s?
I had a Health class last year where I wrote two detailed essays, both scoring 75% and 78%. My assessor gave me really good positive feedback, and highlighted what I did right, but didn't follow through on what I could have done to get even better. I was very happy with my grades, but I needed to ask and clarify what I could have done even better
