Why do I suck at school?

It's alright to slip a little in grade 8, even 9 and 10. Right now, take the time to adjust to high school in general, even if you are planning on post-secondary education most colleges and unis only look at your grade 12 marks, not many even look at your grade 11 marks. I'm not saying you shouldn't try hard, but don't feel too bad; your marks are not terrible, and it can be hard to adjust from elementary to high school, but you will get used to it. I would suggest you to get a tutor if you are really concerned, or join some after school peer tutoring programs, if your school has any.