Honestly, if I were you, I simply wouldn't list my high school dates on my CV. Lots of applications don't, and I've never seen this be a problem or concern. Given that you now are getting an undergraduate degree, the graduate schools won't care much about it. I doubt anyone will pay much attention if you do write in your date of high school graduation and simply don't explain the gap, but why even bring up the topic?
EDIT: Just to clarify, I wasn't suggest you list your high school without dates, but that you just leave it off entirely. I would say you definitely don't want to list it once you've entered grad school. When applying to grad school as an undergrad, I think it's fine to leave it there, but don't expect anyone to think it's important.