I am going through the same thing. I am an intern also and i would just say to give them a notice of when your last day of work will be. Just explain that you will be leaving off to school in the fall and thank them for the opportunity.
I'm currently enjoying my time as an intern for a local technology company, but soon school will be starting up. As an incoming freshman, I'm not sure when I should begin to wrap things up at work... what do most upperclassmen do when involved in their own internships?