Uh, this really isn't true for med school admissions.
Writing a lot about what you did in high school while not doing much in college is a red flag. Bringing up something you did in high school that adds to your application or to who you are as a person is not a red flag, and I've never heard a committee member say "omg how dare he post that he did this one thing in high school as one of his 15 activities." You don't need to pad your application, but I've seen many successful applicants who had a significant activity from HS.