Hold the steel balls in separate hands. Colliding them together at significant velocities will result in a clanking sound, but other than that its not obvious where the kinetic energy of the balls goes after the collision. But the energy must go somewhere! Be sure not to pinch your fingers when colliding the steel balls.

Now collide the balls with the sheet of paper sandwiched in between (use the stand to hold it up so both your hands are free). In order for this to work you will need to give the balls a significant initial velocity and make sure they collide head on.  If done correctly with enough kinetic energy, the collision will literally burn a small hole in the piece of paper. Although the hole may not be visible from the back of class, the paper can easily be passed around or shown on the projector and the smell of the small fire will be very noticeable when close to it.