I worked with a girls high school for a while. We used standard Serratus packs.
As another answer pointed out, the critical dimension is the distance from the shoulder to the hip bones. Girls at the school ranged from about 80 pounds to about 160; from ones that could play Tinkerbell in Peter Pan, to Brunehilde in DasDie Walküre We had to keep small packs in stock for girls that were short torsoed.
Many packs now have a means where you can choose therewhere the shoulder straps attach to the body of the pack, usually by unthreading the Y and running a main support strap under more or fewer bands on the pack. While a PITA to do, this made it possible to fit the pack properly to the kids.