Is you daughter being seen at a major pediatric medical center? I think if I were you, I might take her out of school until you can get to the bottom of this. I don't think it is acceptable to allow a child to regularly pass out at school - it seems to me that this is too risky a situation.
While your daughter was unconscious for an hour at school were paramedics called? Was her heart rate monitored during this time? I would ask for a referal to a pediatric cardiac Electrophysiologist (heart rhythm specialist) and soon! Check out the SADS website for info on heart rhythm disorders affecting the young. Do you have any family history of sudden deaths?
I would suggest that you visit a neurologist to rule out narcolepsy or a similar condition called cataplexy. These conditions can render the sufferer unconcious, or apparently unconcious, for long periods and can be charactarized by extremely hard to detect vital signs. Breathing becomes shallow, heart rate is reduced and weak, and often a pulse/blood pressure is undetectable, though this is not a "requirement."
It could be that your child is experiencing this sort of neurologic reaction that is being diagnosed as a heart rhythm disturbance because of the effect of the "fainting" on the vital signs.
Good luck!