When I was in high school, I was in a marching band that was over 350 people. I learned what it meant to be in phase and out of phase because we all had to march in perfect synchrony. Unfortunately, none of my students have ever been in a marching band so this is as close as I can get without teaching them marching. A student commented, "They look like little soldiers!".
We also talked about the transfer of energy through the table and how interference must be happening in order for the metronomes to "know" about each other. Towards the end you have 31 metronomes in perfect synchrony or "phase" and one pink metronome that is exactly 180 degrees out of phase with the rest. Eventually he syncs up. From this video we learned a new synonym for phase that we didn't use before, but will now use forever: synchronization.