The mechanism of gravity is not only unproven but still speculative. Your notion of proving by dropping an object is akin to proving evolution by pointing to a human. Evolution theory is critical to modern biology so the theory you think is too speculative for high schools is taught as the basis for medical schools subjects. There are all kinds of experiments demonstrating evolution as true and you can do your own research because its been my experiance that it is wasted effort on my part to try to teach people who don't want to learn.
Gravity is the phenomenon whereby things fall down (let's spare each other the details). I see a thing fall down, I see gravity exists. Evolution is the phenomenon whereby girls and boys of a species fool around and another species emerges. I see two lizards produce a fish, I see evolution exists. I see a human, like you suggest, (or a fish, or a dead lizard) and I know a human exists, but I know nothing about evolution.
If experiments that prove that evolution exists, exist, then teach them. What are they? I never heard of one, not did my stepson.
If the evolution theory is useful for medical students, teach it to medical students. Do not teach it to high schoolers who need to understand what science is in general; or teach it as a speculation useful in medicine.