You say that the daycare says she is barking and growling at the other dogs? How are the other dogs reacting? What causes the confrontation? Is it just barking and growling, or is it actual dominant behavior.
You say she is fine around dogs she knows. I know some daycare's have dogs in specific groups so no new dogs are introduced (which is really the best way to do it, b/c packs constantly changing with different dogs can cause problems) Perhaps she can get into a set group like that.
I take it she did just fine in the obediance classes. Do you know what kind of group she is in at the dog care? Is it just puppies, or adults and puppies alike? If she is just barking and growling at the other dogs, w/o following up with any other behavior, I wouldn't necessarily say it's a dominance issue. She may just have issues with dogs she doesn't know well. Or, now that she's older, she may not tolerate some of the behavior of the other dogs as well as she used to. Barking and growling is corrective/warning behavior. It's hard to get a good idea of what is going on w/o actually seeing the dog in action, and I'm sure you can't really get the whole picture from the day care people either.
Someone posted an excellent article awhile back that sounds like it might be very similar to this situation. I'll see if I can find it, but I'm not sure which section it's in.