Detecting atmospheric voltages on high altitude balloon
Sounds like a really cool high school experiment.

Detecting the voltage with a static pickup is nearly impossible because just about any means to measure the voltage will have lower impedance than two electrodes sitting in air.  Even a really high impedance voltmeter will bleed off charge without it collecting.

The trick is to keep flipping the electrodes mechanically and look at the amplitude of the AC signal.  This is what a field mill does.  It's usually two electrodes on a spinning cylinder, and is how electric field strength of air for purposes like lightning research is measured.  I expect you'll find lots of info using "field mill lightning" as a search term.

Try that, then come back here if you have more questions.