I have 3 daughters, 13, 10 and 3, and another baby on the way. I was told by several pediatricians that it's hereditary, which I believe to be very true. My mom had accidents well up to puberty, as did I. My 13 year old still has an occasional accident, which has gotten better over time. She wore Goodnights up until her 13th birthday, then the doctor gave her the "bed wetting pill" which worked great. The 10 year old has never had a problem, but the 3 year old, fully potty trained still wears an overnight pull-up at night. The bed wetting pill has it's side effects, that's why the doctor and I decided to wait as long as possible to give it to the oldest. She'll grow out of it, she just needs a little time. If she's a hereditary bed wetter, no bells, whistles, alarms, waking her up in the middle of the night, cutting off liquids will stop it completely. Trust me, we tried it all! We found the Goodnights and waterproof mattress pad to work the best. It's just her bladder not growing as fast as she is or her urethra getting the signal or strength it needs for her to wake up with the urge to go. Hope this helps you! Best of luck! (The Goodnights also made it possible for her to stay at a friend's house or Grandparents overnight without being embarrassed with a wet bed in the morning, and explaining to her it was something she couldn't help and would outgrow, helped her understand it wasn't her fault.)
Also, reading Michelle's response... YES! Tell her teachers! They are more than willing and understanding to put a couple extra bathroom passes in her desk that she can use without having to disrupt the class or feel embarrassed that she had to raise her hand in front of everyone to go potty again. Having an accident at school because your teacher doesn't believe you or says no because you just went is very embarrassing, I had that happen to me a couple of times growing up. Times have changed so much, and I've found teachers are so accepting and appreciative that you let them know about the problem. Once again, I hope this helps, and I didn't ramble on too much!