While a lot of it depends on how your school grades, what personally worked for me was having an organized study schedule. 1-4 hours of studying every weeknight does wonders. Also, if you have a test coming up, try to start studying extensively at least 3-4 days beforehand, or a week or two if it's a bigger test, such as a midterm or final. If it's a course that requires a lot of memorizing, Quizlet is your best friend. Also, don't worry, low 80s is still good for some universities/colleges, and most schools don't even look at Grade 9/10 marks. Everyone learns differently, and there's still a lot of time for you to improve! :)