First off, it's really your call as to what you choose to wear, but it's worth taking into account all sides of the safety argument.
Your statistics sound a bit off to me (and I have nothing against helmets BTW, having spent the vast majority of my flying time wearing one). As well as protecting your head from impact, the helmets extra mass will also add to the inertia of your head in a sudden decelleration. This will increase the risk of you suffering neck injuries considerably, and neck injuries have featured fairly often in the accident stats.
Your instructor also has a say in this. He is captain of the aircraft, so has the perfect right to tell you what is acceptable to him or not. He may well be concerned that your helmet is more likely to cause him a head injury in any incident, as an example as to why he feels it's innappropriate.
Finally, I worked on the design of parachute systems for many years and there is no way on this earth that a hand deployed parachute will deploy and decellerate a human body to a safe speed when exiting an aircraft at 200ft. I've done a low level forced deployment static line jump and even at 400ft, with the parachute literally forced out by the deployment sock, it only just had time to get me to a safe speed before hitting the water. It's also very difficult to climb out of an out-of-control light aircraft, and if it's in control your chances of survival are better from doing a forced landing.
Anyway, having said all that, the vast majority of accidents occur when taking off or landing, so a parachute wouldn't actually make much sense in a normal GA type.
If you're doing aeros, where loss of control or aircraft failure at altitude is far more likely, then I can see the merit in wearing one. Similarly when gliding, where you are deliberately flying in close proximity to other aircraft, often in pretty turbulent air.