When I teach balance I draw a simple seesaw on the board (triangle with a line on it). I then draw two equal sized people on either end and tell them this is them and their best friend, and I tell them this is symmetrical balance. Then I draw a large person on one side, two smaller people on the other. This is me on one side, two of them on the other (this elicits giggles), and this is symmetrical balance. To do radial I then bring out "Fog Warning" by Winslow Homer (this is my favorite one for radial). We discuss what they see, and they are always excited to see that their eye really does travel around the picture between the three main objects. You're right - 15 minutes isn't very long to teach a lesson to 9th graders.
I interviewed for a high school art position on Friday and got a call to come back on Monday to teach a 15 minute lesson on Balance I can teach any aspect of the concept that I want. The class is a studio art class of 26 students (all 9th graders), they already sit in a group setting at tables. I don't know exactly what they will be looking for but I suppose the usual, control of the class students engaged in the lesson........ but 15 minutes is not very long. I have a few ideas but wanted to tap the great ideas generated from this group.