Don't give up.
Firstly, ask for help!
Do your teachers know that you are struggling?
Tell them your dilemma, I think this will take a lot of the stress away. if you are completely, lost spend time going back and reviewing the topics. Oh and listen tentatively in class, and if you unsure just ask the teacher (you could even stay back after class if you are not too confident)
If you might also consider a good tutor to help you. Don't skip chapters, please go back and review them.
I might be scary, but tell someone you are struggling (teacher, friend, classmate) , and maybe after school, they can help you out.
Don't forget there's ample support out there,
So seek help, be dedicated and give it best shot in your subjects, and I wish for you that you get the results that reflect your hard work and endeavours.