Can you include those hours? You can include whatever you want. Will it hold much/if any weight? No, not really.
Unless you are continuing something you started in high school into college, stand-alone activities from HS won't hold weight for med school admissions. Schools want to see what you have done in the past 3-4 years in college, not what you did in high school. The only exception I see to this is if you go to college in the same area as your high school and you continued the research or volunteering through college. Then you can lump the high school hours in.
If you're a freshman in college, stop thinking about AMCAS. Go out and do things that you're interested in and get clinical experience and research experience in college. If in a couple years you sit down to fill out your AMCAS work/activities and you feel the need to draw from high school experiences, you can do so if you'd like, but know that it doesn't make up for any holes in your college activities.
I do understand where you're coming from, though, as I shadowed and volunteered through all 4 years of high school. Instead of putting them in the work/activities section, I talked about an experience from shadowing in high school in my PS, tying it into why I wanted to go into medicine. Consider that as an option for those experiences. It's also good fodder for interviews, when you will inevitably be asked "why medicine?"
The experiences were valuable I'm sure. They just won't do you any benefit in the work/activities section of AMCAS. Show what you have done in college.
