We've been battling this forever as well with my daughter who is now 11.5. We took her to a pediatric urologist and they measured all her parts in an ultra sound to make sure everything was okay there, then did an MRI (I think- I'm not medically inclined) and they showed me all the poop backed up through her system. It was horrible and I had no idea. She was clearly at an age where I wasn't looking at or having any reason to give it a second thought. She thought it was normal for poop to be hard and never saud anything to me. This was in the 3rd grade. 2 years later we are still dealing with it. They say it could take years for her muscles to regain strength because they are so stretched out they can't hold the pee in. She wears an alarm every night, we cut out a lot of dairy, added fruits and veggies to every snack, and she is supposed to take Miralax daily. Hope that helps.
Homeschooling mom of DD 10/01, DS 5/11, and expecting #3 June 2013.