Here's some I found on BullyWiki:
Melody -

Shannon Amabile
Mr. Oh (YumYum Market manager) -

Andrew Pang
Mandy -

Elena Franklin (already listed, I know, but the picture isn't showing up)
Russell -

Cody Melton
Tom -

Connor Paolo
Mr. Morrati (Happy Mullet barber) -

Vincenzo Sanseviero
Troy -

Evan Weinstein
Davis -

Geoffrey Wigdor
Here's the BullyWiki Voice Artists list:
http://bullygame.wikia.com/wiki/Category:Voice_Artists It has short bios on everyone.
Something weird I noticed while looked at all the bios was that nearly every voice actor listed had played a part on Law & Order before their role on Bully. I guess the guys over at Rockstar like Law & Order?