OK, I have to admit, right up front, I don't understand String Theory. It's REALLY heavy-duty theoretical physics, of the sort that has no way of being introduced into the average high school physics clasroom. Still, some years, a physics teacher might have 1 or 2 students who have an interest in learning more. A good starting place is Discovery Space, which has
a brief overview of the theory.It's worth reading for the video alone.