I'm surprised Schwinn does not provide a longer seat post. I have
asked also and they didn't have one.
Solution 1: File a notch across the bottom of the seat post
(parallel to the bolt holes) Put the seat post above the
bolt (notch fits over bolt to prevent turning) and
tighten bolt. The friction keeps it from coming out.
This will get you an extra inch. (Schwinn will not recommend
Solution 2: Buy a 13" Schwinn post (or 18") from Tom Miller for $20
plus shipping. This will get you an extra 5 inches.