If you're talking about a pannier that has double shoulder straps and turns into a true school book bag, they aren't a super popular item. There are a few, but they aren't easy to find.

If she can deal with a shoulder strap like this...
http://toolsandtoys.net/wp-content/u...lerCity-12.jpg

your options are nearly limitless. There are hundreds of options in every price range and color. They lock onto a rear rack, then pop off and carry like a shoulder bag.

Problem if they tend to be smaller than a true back pack. I have a pair of 18 liter bags that are perfect for commuting with a change of clothes. But if I used them for school each one would probably hold 2 books and a notebook comfortably. I couldn't put 5 text books, computer, and a science project in one.