So right now I am in the last but one class and always got through school without much trouble, I had understood most things by learning them in class or study a bit on my way to school or during breaks.
However, suddenly things started going south. I can not keep up in classes because I do not comprehend the subject, when asking the teachers, most of them fail miserably at explaining. When I try to study at home, I can not concentrate on my studies, my mind drifts off immediately and I lose focus. Telling myself just how important it is does not work.
So I guess the whole post comes down to this:
How do you keep concentrating on subjects that do not fascinate you in the least?

This is happening to me too. When the content is harder, you have to put forth more of an effort than you are used to, especially for something you don't care about. For me, it's easy to care, but that doesn't really help me at all. It doesn't make it easier to do when I realize how great it is.

You have to learn it on your own if everyone you ask can't explain it to you. This by itself is a skill, one I do not have. Learning by yourself can be tough, but ultimately it gets the same effect. If you can't focus, I can't really help you. I have the same problem and I have yet to solve it. I tried eliminating all external distractions, but I still get distracted by my own mind. I hope you figure it out somehow.

What I do is just remember what the teacher is talking about. I pretend to be a detective and put the pieces together.

I try to find a connection between the subject and an interesting fact or reference to it.

Well, I don't usually concentrate in subjects I don't need, that is,they're not 'pre-requisites' for the university course I want to take. Example Chemistry, I have no clue about anything, so I don't have any choice but to go back to square one and work on from there in my spare time.