High volt kit

A big enough spark??

You don't say how high of a voltage or what amperage capability(why am I not surprised), but there are many ways to demonstrate it. Electroman mentioned a good one: jump a spark if the voltage is high enough. Depending on the voltage you could create a simple circuit with some resistors and a neon bulb and have it light BUT you don't say what voltage.

But it sounds like your teacher won't accept a voltmeter reading (even if you put the appropriate load on your output?). At that point, I would suggest (as a joke) that you have the teacher hold the output leads ... but its just possible someone here would do that with a dangerous voltage so DO NOT!