Agree with the posts above.

While very exceptional candidates do get in from undergrad in small numbers (and some programs, again very competitively, have deferred entry options), and some lower-tiered programs will accept more straight from undergrad, most b-schools do want you to have full-time post-undergrad experience.
Part of the reason for this-- much of your learning from b-school actually comes from your CLASSMATES; it's not just the curriculum/professors.

B-schools aim to create a microcosm of the world, if you will, within the classroom, and much learning through case studies and discussions, involves hearing perspectives, stories and experiences from those in the room with you. So they value a diversity of experiences from the professional workplace. Additionally, it's tougher to apply what you learn during the MBA classes if you don't have much experience in the work place as a point of reference.
But it's good to plan ahead, and work on your leadership and your people and project management experience; community involvement, etc. And the GMAT if you haven't yet... statistically, those right out of school do better on the GMAT, and scores are valid 5 years!
Good luck!