I am writing a novel in which a group of children find a secret room
that goes to the other side of the teachers' lounge.  They are able to
hear all the teachers' conversations, but the teachers can not hear
them.  The room would've been built at the time of the Civil War. 
Would it have been possible then to have a room that was sound-proof
on one side with normal acoustics on the other?  What materials would've been used?

Here's how I'd do it:

Soundproof the wall between the two rooms thoroughly, so no sound can
get from either room to the other. The attach an (or several, for
better sound transmission) 'ear-trumpet' style device, with the wide
end in the room to be snooped on. Then baffle the thin end (in the
children's snooping room) so they can listen by putting their heads
near the tube, and no noise will get into the first room (unless the
make very loud noises!). This works because the wide end of the
trumpet gathers sound very efficiently, and 'concentrates' it down the
tube. The thin end, on the other hand, hardly gathers any noise from
the surrounding air.

if you want a more detailed, physics-style explanation, let me know. 

in case you've not seen an ear-trumpet, here's one:
http://www.pbs.org/wnet/soundandfury/cochlear/images/amplify.jpg

hope this helps,

jim whitson