The topic of playing multiple sports comes up every year. Bottom line is if the girl is athletic, uses proper mechanics in each sport and keeps herself in excellent shape using a quality strength & conditioning program, there's no reason she can't play every high school sport available to her - such as volleyball in the fall, basketball in the winter, then softball in the spring.
I've heard endless excuses, but none of them really have any merit. IMO, most injuries can be traced to improper mechanics, improper warm-up, lack of conditioning/stretching, etc. Also, not every kid is going to have the strength and stamina to "do it all". Unfortunately, thats due to genetic traits handed down from mom and dad.
Think about it - would you want YOUR parents preventing YOU from playing a high school sport you have a passion for? Of course, assuming your grades are acceptable. And shame on the coaches who are selfish enough to just want a kid to play one sport - theirs. That one REALLY gripes me!
Keep the grades up - stay in shape - and play everything you're capable of! You only get one shot at high school sports!