What kind of plane are you flying? If it's a flat bottomed wing trainer, I'd check the balance again carefully, as even with downward incidence on the wing, they will still tend to take off. 250 feet is a very long takeoff run. And half up elevator, are you sure your engine is running OK? Has a big enough prop? And once again, are you sure the plane is balanced correctly? I'm sure you're in good hands with your instructor, but what you're saying is not good.....

Andy