Ask.

I can’t focus on anything for more than a few minutes without either thinking of him, or something involving him. I can’t stop. I need to, but I can’t.
Advice?

Unfortunately, there is no legitimate way to avoid this.  He’s obviously made an impact on your life large enough to fill your mind with the thought of him so often.  I’ve been down this road, and one of the best things to do is to just remind yourself that there are more important things to do.  I sound like a hypocrite when I say this because I honestly don’t give a shit about school, but concentrate on your school work, and remind yourself that a year from now, you aren’t going to be concerned about him anymore.  It’ll pass.  Just keep up with what needs to be done.  When you’re heartbroken or smitten or absolutely crazy about someone, time doesn’t stop to wait for you to get your act together.  Keep up with your life.