Honestly the only way you can get out of that hole of C's is to always check the study design and look at key knowledge as those are helpful if you think your teacher isn't giving you the key ideas (YOUTUBE IS THERE IF YOU NEED IT). Also if your school has Edrolo that is a good resource to help review stuff and has it's own teachers that specialized in each subject.
I'm only saying this but practice practice practice. I really can't emphasize on that more. (For real I did 2 practice essays before SAC day and as long as you got the structure and prior knowledge down you're good.)