I'm doing very poorly in school, what can I do?

So I'm currently in the 11th grade and I am STRUGGLING with my grades. I always make sure to study at least a week in advance and I study and practice hard. But it never repays. I always study my heart out for even a C but I end up with Ds and Fs and it's extremely frustrating because this is happening with 4 of my subjects: Physics, Biology, Accounting and Maths. I don't know what to even do anymore because I'm pretty sure I play my part right. My teachers never have time for remedial classes and I'm always on my own. My parents aren't interested whatsoever in my schoolwork, so I've been begging since 10th grade for a tutor or just remedial classes...but no, nothing. They always say "We'll try tomorrow", every single day. Then once my report comes I'm yelled at and criticised, but I had played my part and worked as hard as I could and yet I still get this treatment. I just don't know anymore, I don't want to see myself repeat the year.

Please help me(If it helps, I'm 15, turning 16 in June so I started school a year early)