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I want to attend either Wharton or Booth. Will a high GMAT balance out my undergrad school? Is this something that will need to be addressed in my application essays?
What are you going to write? "I went to a school that really sucked back in the day, but look at my mighty GMAT!" ???
Do not worry about the rank of your school. Who came up with that ranking, anyways? Is this rank an objective measure? If it is not objective, it does not matter that much.
What did you achieve with the education you got? This is what matters.
I am a prime example of a person who went to a no-name undergrad (Chita State Pedagogical Institute ring a bell?); I majored in Chinese and English, and minored in Education. My major allowed me to reach the ranks of United Nations translators in Chinese, Russian and English, and my minor allowed me to be charge of a private university, a major international school (all in China) and my own enterprise (in Russia).
Business schools do not punish applicants for not having attended a big name undergrad school. Although it does help a little to have a big name on your resume under "Education", this single factor will not be decisive when they evaluate your candidature.
Remember, all factors TOGETHER matter and this combination is something that should be the core of your application strategy.
Good luck!