I have my girls do pommel horse, as well as supports on rings. I'm in a rush right now, but I'll give you the full list of shoulder conditioning I do with my girls later.
EDIT: Ok, here's what my girls do for shoulder strength:
-Leg cuts on pommel horse. Alternating sides, and they should (try to) swing, like proper leg cuts. In reality, this will take them awhile to get, and until they get the idea, there are numerous pommel-horse drills that will help them with it. One is to rock from side to side in a push-up position, lifting one hand at a time. This both strengthens their shoulders and gets them used to shifting their weight to one hand. If your gym has a boys' team, it probably wouldn't be a bad idea to get the boys' coaches to help teach the girls to swing pommels.
-Supports in straight body or pike on rings. This works great for us, because we have a set of low rings right near our bars. It's pretty self-explanatory really -- they hold themselves in a support on the rings.
-Tucked planches/tucked Vs. They can do these on a pommel horse, p-bars, parallets, or if they're strong enough (which it sounds like they aren't), on flat floor. They hold a tuck support, then rock it forward as far as they can (planche) and hold for 15-30 seconds. Then they rock back as far as they can and hold for 15-30 seconds.
-Prone planches. From a push up position, they pull their shoulders forward as far as they can past their hands while keeping a straight body and not falling and hold for 20 seconds, then go back. I usually have them do two or three sets of these at a time
-Back supports on floor. Like a pushup position, but reversed, so their feet are in front. Just holding this position is quite a workout; I typically have my girls do two or three sets of a minute of this.
They also do some press work during warm up. Endo rolls and back extension rolls (and press handstands, if they can do them) are great for working shoulders, and we incorporate them into our daily warm-ups.