End of my senior year of high school I did a program with my local osteopathic school and it heavily influenced me to become a doctor. Can I include that in my application this upcoming cycle or is high school activities a strict no?
This is correct. Do not include it in your activities.
As DO2015 said, I think it has a place in your essays - especially in your “Why DO” essay. Essay topics can cover anything at any time. I personally talked about a high school shadowing experience that pushed me down the path of medicine in my medical personal statement.