Sorry to hear about your daughter's problems. I do sympathise as I had similar problems myself with recurrent UTIs / kidney infections from the age of 5 through to 12. The first time was really hideous, passing blood etc. This was before trimethoprim had come on the market and I was put on another antibiotic daily, although I don't recollect that it was very successful. We lived in the US for a while and when there, they had an exploratory rootle about - and the best they could come up with was similar to what beetle73 describes: a malformed valve was allowing reflux of urine back from the bladder to the kidneys and they just hoped I would grow out of it. This wasn't definite however, just their best guess.
When we returned to the UK, I was lucky to be referred to the Professor who had developed trimethoprim and a daily low dose of this seemed, in general, to stop the infections. I still did morning MSU samples, at first monthly, then with decreasing frequency, stopping at 16. The last few years, I was only doing them twice a year. I stopped the medication at 12. The GP told my mother (whilst... ahem... looking embarrassed) that the infections might return 'when I got married' . Although I have had UTIs as an adult, there have only been about three or four, mainly related to my mode of contraception (can't use a diaphragm, for instance). I have been lucky in late pregnancy (as I now am) not to get an infection, but have been taking cranberry tablets for a while. If / when I do get a UTI these days, it's always nasty, with bleeding - so if it's out of hours, I don't bother p*ssing about with the GP, but have gone to A&E.
Your comments about the schooltrip rang bells, as my mother found it very difficult to get through to teachers, dinner ladies etc about my need to go to the toilet immediately and consume large quantities of water / dilute squash. More than you would think. My school had some bizarre Dickensian rules around lunchtimes and you weren't allowed to drink anything until after you'd had your meal; there were no drinking fountains.
Even now, I have a very high fluid intake and am very sensitive to my urine becoming more concentrated than usual (if I'm going away, I usually have some sodium citrate sachets in my toilet bag). If I do a sample now, it usually looks like I've just run the tap into the jar - good for the skin though!
Another thing that was suggested was to go to the loo every hour to squeeze some out, even if I didn't have the urge. I'm not sure I ever followed that through, however, because I was usually in the middle of doing something that I didn't want to leave. I also hated being treated like some kind of invalid, even if it was for my own benefit.
I have heard that changing your washing powder and not using soaps in the crotch area can help.
Anyway. Sorry this is a bit long - but really, if it's gone on this long, I'm surprised the GP hasn't suggested referring her to a urologist before now. I'd jump up and down and have a right hissy fit with him / her if they're less than co-operative. It's so debilitating. Going to the loo first thing in the morning was sooooo painful, I used to lie on the floor for 20 minutes afterwards waiting for the pain to subside (on the upside - it has left me with pelvic floor muscles that could crack a walnut... )