Rampid,
I'm a dedicated batting coach. For a younger 12U, average size, I recommend staying at 31", especially if she has any kind of coaching that is helping her develop a proper swing. A -11 or -12 is often appropriate for this age and size.
If you live anywhere near the Dayton/Cincinnati area, there is a bat factory in Springboro which sells blemished versions of the Rawlings LiquidMetal bat for half price. This is a great savings potential on an above-average bat if a -11 suits you. It is called Custom Bat Works.
Details:
Jerry Salyers runs the outlet store -- he's very friendly and helpful. They are open from roughly 9:00 - 5:00 weekdays, 9-3 Saturdays, 1-3 Sundays, but call first, toll free at 1-888-228-2891 (1-888-batbuy1). Jerry might flex his hours to meet your needs if you ask.
Address: 200 S. Pioneer Blvd. Springboro, OH 45066
Directions: From Cincinnati, take I-75 north to Exit 38, which is route 73. Turn right to go East. Just past Kroger, turn right onto Pioneer. Go down a ways, and just past a big baseball field with lights (Brandt Field), turn right into their driveway. You'll see the outlet store to the right of the factory, meaning straight up the driveway.
If you go there, please tell Jerry Coach Dan sent you to him. (I am not associated with them and don't get a kick-back.)
The Rawlings Liquidmetal is excellent, and a 33/22 size is likely to work great for the stronger high school players, 32/21 for average ones, and 31/20 for less strong grade school youngsters. Be sure you get the Fastpitch version -- it says fastpitch on it. If it's not a -11, it's for baseball or slowpitch.
Dan