Well, I don’t think there’s any way of figuring out the spelling, because according to the (supposedly) original script (http://www.hundland.com/scripts/FerrisBuellersDayOff.txt), on page 189, it’s:
BOY: What’s your name?
JEANIE: Jean. What’s yours?
BOY: Garth Volbeck.
And that’s all these grubby paws could dig up. I guess the only way to
know is call ol’ Johnny H. himself. IMDb probably has has his agent’s
number, but I’m too cheap to pay the long distance.
Good luck,
Bob Weisz