There's nothing stopping you from listing it, but would listing it help your application in any way? That's what you need to determine.
While in high school, you probably didn't have college level biology, chemistry, physics, orgo, p-chem, etc etc etc. Some MD and/or PhD researcher with publications and RO1's might not be too impressed with your high school research. Like most research experiences, think about what your involvement was. Did you plan and design experiments or did you just follow the directions? If you're quizzed on this research are you going to give intelligent, articulate answers? What if your interviewer throws out a hypothetical question about your research? Better come up with something other than "I don't know" or worse "um......"
Your HS research might have been great and rivaled that of one in college, but we don't know that and neither will adcoms. Therefore you'll have to make an extra effort in your app to convey the notion that your research was worthwhile. HS stuff will be up against some bias (and for good reason). That's what you're facing.
Basically, your high school research will have to be pretty impressive to matter.
Good luck!
-X