Okay, in my opinion the "Nothing From High School" Rule works like this: if it's something you happened to do on the side, that you could still do now, it might be fine.* If it's high school related (i.e. anything to do with school, clubs, sports, awards, community service, etc.) cut it out as soon as you get to college. An instrument is kind of a lifelong thing, and doesn't actually relate to your high school, so I think it's fine. Now, if you played french horn in the band, I might leave that off.
I haven't taken piano lessons since high school, but I took for about 10 years and still play at events occasionally, so I would include it on my resume (if I had room, the only reason I don't is for lack of space.)
*You will always want to get opinions just in case.