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.