What can I do about my focussing in school?

Well, this past month in g 12 has been the hardest on me ever. I zone out constantly, or get songs stuck in my head, and think about other things in cl an durring examples. I am failing 1/2 of my cles, and can't focus on anything. When my mind gots off in another direction, it is usuaally about girls, , etc. Etc. And I also... Fantasize about the future, like marrying someone and having kids ect etc. This is not normal for me. As a dude, I never thought about any of this ever beforean expecially the marrage part. I also think of and women in a ual matter lots. I haave never been laid or even had a girlfriend. So, what can I do? Abd m not the type of pperson to go out and get some straange reaadiway or anything. Thanks