Hey Swis, it's very normal for a teen's mind to be consumed by a lot of different things.  Thinking about girls, sex, and playing video games can seem a lot more interesting than studying for a test.  But your education is important, and you need to make time to focus on it.  If you're having trouble keeping your mind from wandering while you're trying to study, I'd recommend a change of scenery.  Try to set up an area of your house, that can be strictly used for your study area.  This will get you away from the distractions of studying near a t.v., computer games, posters on your wall, etc.  If you can't have a place like this in your house, try your local library.  Libraries are great places to focus, they're quiet and filled with other people who have their head buried in books.  Once you establish this study area, you'll train your mind that when you're here, it's down to business and time to study.  Oh, and turn your phone off while you're in this study area!

Good Luck