Top Law Schools

A few questions:
How do you withdraw from a school?
Why do people withdraw (ie can't you just not respond to their admission offer)?
Can you withdraw an application that is still pending?

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It's also good manners to withdraw so the school can move on to offer spots to the next applicants down their list. Someone else is on the waitlist, praying a spot for them opens up, and you're just being a dick by holding onto it out of laziness.