I had this problem in middle and high school and even some in college. I had a class that was months solely on the US Constitution and all amendments and some proposed amendments. Terrible droning lectures in that class, it was the dullest thing ever. I am sorry I have no great advice on staying focused in that environment.
My only thought is when you have choice of classes and teachers ask around to find the lively ones. Even If it's an elective you aren't necessarily excited about, a lively, interesting teacher will make it worth your time.