The traditional wisdom on this issue is that if you continue an activity from high school and into college, then you're free to include your high school time when writing about your involvement with the activity. If you stop the activity while in HS, then it probably isn't very relevant.
The theory is this: Your activities from high school helped you get into college, and quite frankly, by the time you apply to med school are pretty long ago. Hopefully you've been active enough while in college that including HS stuff will seem pretty laughable.
There are definitely exceptions. I'm just expressing my own thoughts about it after doing research about this on SDN and various med school websites
Keep in mind that with shadowing, it isn't a competition to amass the highest number of hours. Most premeds only have like 50 hours of shadowing, which is easy enough to obtain in a few weeks over a summer. If you have 300 hours, adcoms are going to wonder why you spent so much time standing in a corner watching someone else work instead of volunteering! Shadowing is important to obtain but a great deal of hours isn't so important