self-esteem
The best description of true self-esteem that I've ever heard was, "Self-esteem is the reputation that you have with yourself." In other words, do you see yourself persevering, succeeding, getting along with others, handling challenges without falling apart?
There are often two related problems. One, that kids don't see themselves accurately, either because they're feeling depressed or stressed or some external factor is causing them to believe negatively about themselves (and they're vulnerable enough to believe it). The other is that the child might be genuinely having a hard time with school, or life in general, and is accurately perceiving that struggle, and doesn't trust himself or herself to get through it.
If it's that the kids don't see themselves accurately, there are a variety of ways to go about getting them to change their perspective. The book Learned Optimism by Martin Seligman is a particularly good choice (it also has some choice words about the "self-esteem" movement vs. actually teaching kids to see themselves in a positive, REALISTIC light)
But if it's because they're struggling in some way (socially, academically, etc.) then the best way to improve their self-esteem is to help them succeed in those difficult areas. Certain things are likely to really make a kid feel down, such as not being able to read at an age where everyone else is a fluent reader, or not having any friends. These are basic developmental skills that must be achieved if the child is really going to feel positive about himself or herself.
So if you first figure out where the breakdown is for these girls, that might suggest a clearer way to go about helping them.