Qualifications
Plenty of sound mixers have simply worked their way up in the business—usually starting out as a boom operator and moving up the ladder if they can prove themselves to be knowledgeable and dependable—but getting a college degree would not be for naught. It's important that a sound mixer understand the science of sound and have an extensive background in technical audio knowledge, much of which can be picked up at a trade school or in a sound technology course.
You may have to major in something silly like "business" in order to graduate, but hopefully you'll never have to resort to pursuing a career that makes direct use of it. God forbid you should make some serious money.