When Mika first went to puppy classes, she was quivering under the chairs as well. She'd only come out when it was our turn to do something. By the end of the 6 or 7 week class, she was out from behind the chairs. Even though she was so shy, she was always excited to go.
We've continued on with that club and now do agility as well. She can hardly wait to get there. She's still not big on other dogs, but will let them approach and some she even decides are trustworthy and will approach them first (not very often). She is best friends with our neighbours dog and this is the only dog she really plays with. When they brought him home as a puppy, he harassed her until she finally learned to play, now she loves him and they play like crazy. Most other dogs in the neighbourhood are more like "acquaintances". She sniffs them and they wonder around together while the humans talk, but she doesn't play.
She just prefers humans to most other dogs. I'm really glad I didn't give up on her though after those first few classes.