There's no such thing as instant leadership or instant accomplishments. You can't just say "I'm going to get leadership and achievements" and then voila! A few months later it'll be handed to you on a plate simply because you chose to participate.
Participation isn't the same as achievement. With only a few months to a year, you're more likely to only have the chance to sign up and participate. You can't just waltz in and knock off some achievements that you think will impress an adcom. Real achievements take years of cumulative involvement, commitment, sacrifice, and so forth - whether it's community service, arts, athletics, business or anything really.
If you're focused on more extracurriculars now because you didn't have much before, it's too late (at least to change your overall candidacy for b-school). Of course, for "life" it's never too late to get involved in something you really believe in right now -- but it should be with the idea that you would be doing this regardless of your b-school plans, so that your sustained level of commitment in that group activity/extracurricular is something you're doing for its own sake (and the byproduct being some notable accomplishments along the way, which more likely than not is a few years, not a few months away).
Again, if there's stuff you want to do outside of work, GO DO IT! But don't do it for the sake of b-school admissions because it's too late. Do it because you want to do it.
As for school choices, it's a decent range, but again my hunch is that HBS is most likely a long shot for you, Wharton a stretch, and the rest are sweet spots. Whether you dip below the top 16 or not is a personal decision.
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