I've taught on this passage several times in youth ministry. It's a great illustration that Jesus gives regarding our responsibility to shine in the darkness. This go around I wanted to do something different though, so I set out three objects: a light bulb, a picture of the moon, and one of those glow-in-the-dark stars you can put on your ceiling.
Then I went through something along these lines:
This bulb, when illuminated, can light up a whole room. But can this bulb, by itself, produce light? No. What does it need to shine? (Answer - it needs to be plugged in to a power source.)
The moon, when it's full, can be a source of light in the darkest night. But does the moon, by itself, produce light? No. What does it need to shine? (Answer - it needs to be tilted toward the sun so it can reflect its light.)
This star, when you turn off the lights, glows a comforting light. But how does this star get its glow? Does it, by itself, produce light? No. What does it need to shine? (Answer - it needs to be in the light so it can absorb it and then re-emit it as it glows.)
Then you ask students something like this: now what about you? Does a human being, by him/herself, produce light? No. What do we need to shine? (We need God's presence, His power in our lives.)
Then you could ask students what item they think most represents us - are we more like the bulb, needing to be plugged in to the Source of power; are we more like the moon, we need to be turned toward God so we can reflect His light; or are we more like the star, we need to be in God's presence, absorbing His light so we can re-emit it?
When I did this with students, it was awesome! Object lessons can be a powerful tool to use with your group, and every time a student sees a light bulb, one of those stars, or the moon, they may remember that they are called to be the light of the world!!!