While this project works well at school, we realized last year that in order for students to enjoy their completed project at home, we had to design an alternative circuit. We cannot provide Makey Makeys for every student, and it seems unreasonable to expect parents to purchase one (although now with Makey Makey Go the idea is more realistic). Therefore, we designed a simple circuit by adding a coin cell battery, an LED and wire. When students make contact with the rim of any of their operation spaces, the LED lights up.
An alternative to this set up is to use a buzzer instead of an LED (keeping in mind the buzzer must be rated for 3V), but we felt this was a nicer, quieter option. You can see the final piece in the last row of photos below.