Self Sabotage at school and not knowing why

Well, you sound like you already know you can do better and that this is self-sabotage. It seems to me like, by not doing your best, you're avoiding potential disappointment you might feel if you gave it your best and still failed. Which might have worked for you at some point. Nowadays, if I understand you correctly, you're not really avoiding disappointment this way anyway, because you're frustrated knowing you could do better and yet keep yourself failing. The questions to ask yourself are: have you ever had a big success and, if so, what happened then? Did you have a disappointment before and what happened, or what would you expect might happen if you fail after having done your best? If you still don't understand how to change this or motivate yourself, you could see a school counselor if there is one in your school - they should have some experience helping students with these types of problems. Good luck!

My friend and I are dealing with the same thing essentially; Her reasons is the fear I think, that and she's just so drained from family and school she just has no stamina left for exams.
Mine's the same really; all these years and now I just can't be bothered, it's just stupid exams right?
They're important though, just keep reminding yourself:
A)They'll be over soon, and then you'll have summer to do whatever the hell you want and;
B)They're important, and you still have time to pull your socks up and put some time and effort in to do your best.

You're in your final year, right? That's all then. Last few weeks just force yourself to powerhouse through it and you can do it and feel like you earned your summer.