Yeah, I'll agree with Mad Jack. I'm at the University of Arizona, and there's only one (repeat: ONE) guy that the business school ever sent to McKinsey. The catch? This guy did a triple major in Math, MIS, and Finance, had a 4.0, and was insanely driven. The things this guy did while in undergrad was unbelievable, and certainly exceeded the workload of pre-med undergrads and pretty much any other student at the U of A. McKinsey took this guy because he had the drive to exceed all possible expectations and made as many opportunities for himself as he could. For the mortal students, it's highly unlikely that one makes it into a top firm without copious amounts of work experience or an MBA from a top-tier program. Consulting for smaller firms is more within reason, especially if one has knowledge and a skill set within a specific niche that is not prevalent in that firm's pool of consultants.
Part of selling a service is the appeal that is associated with it by consumers. Being able to say they take the best and the brightest makes these firms exceptionally attractive to high profile clients, and so at top firms, there is a profusion of consultants from top business schools, and typically only one or two like my U of A guy.