Growling is communication that usually means "If you keep doing what you're doing I may bite". It doesn't necessarily mean the dog wants to bite; most times it means that they are in a situation that they are very not okay with and would like to get out of without having to bite. (Offensively-aggressive dogs, the ones who.... for lack of better words, enjoy attacking, don't usually give any warning - they are usually totally silent.)
It's very likely that the variety of factors - being away from you, being surrounded by strangers, being in a brand new building, being surrounded by a large number of dogs, and/or new dogs that's she never met before are all stressful things for a fearful dog to experience. It is possible that she might acclimate to the environment with additional exposures, and especially going slow: only meeting staff on one visit and not other dogs, playing with only 1-2 dogs she knows in a visit, meeting 1-2 new dogs who she doesn't know, etc.
But it is also possible that she won't and in that case, have you considered other types of boarding? There are boarding facilities that are not daycares, or pet sitters that either come to your home and house-sit while you are gone, or take your pet into their home, giving your dog a much calmer and less chaotic environment.
It's very likely that the variety of factors - being away from you, being surrounded by strangers, being in a brand new building, being surrounded by a large number of dogs, and/or new dogs that's she never met before are all stressful things for a fearful dog to experience. It is possible that she might acclimate to the environment with additional exposures, and especially going slow: only meeting staff on one visit and not other dogs, playing with only 1-2 dogs she knows in a visit, meeting 1-2 new dogs who she doesn't know, etc.
But it is also possible that she won't and in that case, have you considered other types of boarding? There are boarding facilities that are not daycares, or pet sitters that either come to your home and house-sit while you are gone, or take your pet into their home, giving your dog a much calmer and less chaotic environment.