AMCAS has no written rule that prohibits listing activities from HS (or earlier) on your application. That said, you are limited to 15 so you may want to choose your more recent activities over those from long ago.
Your application should show how you have "tested your interest in medicine". This can be done through volunteerism, shadowing, and/or employment. The point is that you know what medicine is from an adult perspective and have made a decision as an adult to pursue this career path. The number of hours is less important (to me, anyway) than the number of months (you are asked start date & end date) and the average hours per week.
Your application should also show a spirit of volunteerism and ideally this should include non-medical volunteering particiularly if it involves interactions with the poor (free tutoring of K-12 age kids, literacy programs & ESL for adults, food pantries, soup kitchens, homeless shelters, etc). Again, you can list things you did early in life but if you did it at 15 and list nothing of that type beyond that age it looks like a "check the box" activity and not something that is part of your regular schedule because it is important to you as a person to be of service to others.