Sandhya:
This was EXACTLY the thing I was worried about. All my hospital volunteer hours (200+) and accolades (nominee: volunteer of the yr) came during my HS years. Usually one would avoid putting HS experiences, but this is truly an exception. These experiences were crucial in shaping my decision to go into medicine.
The volunteering was brought up (either by interviewer or myself) in all 4 interviews thus far and not one made a negative comment about it being so long ago. In fact, I received some positive feedback that I did take the time to do research, TA, tutor to better 'round' myself during undergrad.
hope this helps!