I know about the square-cube law, and this question is not whether dragons can fly at all, but how high they can fly. This link may help deal with some of the problems that come up.
Edit: Assume their bones are strong enough for whatever is necessary (thanks, Tim Hansen!) and the dragons are able to flap their wings but spend more time gliding. If for your answer to work the dragons' bodies would need to be smaller, feel free to take liberties.
In movies such as How to Train Your Dragon, Eragon, and Pete's Dragon, dragons are able to fly above (at least some) clouds. Knowing that there is less air resistance and less air to breathe, and knowing that the few birds that do fly above the clouds do so for limited time, this seems unrealistic.
Assume that the dragons' bodies are about the size of a horse, the wings are as big as needed, and the dragons take off by leaping off a cliff.
If some sort of lifting gas would help your answer, that's fine too.
How high can dragons fly (and how can they breathe at that height)?