It's that time of year.
Wave-particle duality isn't the easiest concept to convey to high school freshmen (or to anyone. for that matter), but I think I have found a successful analogy that plays on a sort of complementarity that the students have already been using in physics class (and life).
Earlier in the year, when we did projectile motion or force problems, we treated the surface of Earth as if it is flat. Later on, when we looked at gravity and considered Earth orbiting the sun, we treated Earth as if it is a point.
So which is it? Is Earth a plane, or is it a point??? Well, of course, it's neither -- it's (approximately) a sphere. But spheres are difficult to deal with. And when you're really close to a sphere, it looks like a flat plane, and when you're really far from it, it looks like a point. So each of these models is appropriate under certain circumstances.
Likewise, when we observe diffraction or interference or polarization, we treat light as if it is a wave, and when we observe the photoelectric effect, we treat light as if it is a particle. So which is it? Neither -- it's something outside our everyday experience. But the wave model and the particle model are each appropriate under certain circumstances.