Work experience is VERY important. Nearly all top U.S. business schools require (or at least strongly recommend) at least a couple of years of full-time work experience. The reason is that the companies that hire new MBAs don't want to hire completely inexperienced managers; they want young professionals who have at least a little professional "seasoning."
We strongly recommend that you get 2-3 years of full-time experience under your belt before applying. Otherwise, your best hope will be a lower-ranked (below the top 30) school.
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