I've noticed that the first time that I do anything, it takes a little longer. There is anxiety, uncertainty and doubt. Will I do this the right way? Will I be successful? What will happen? Because of all this fear and uncertainty, sometimes it's easier to revert to old behaviors, procrastinate and just simply delay the activity.
The value of business school, or any kind of schooling, is perhaps that it throws you into practicing skills that you might not otherwise think about delving into. We might not leave school as supreme experts in a particular area, but we know a enough about a variety of areas that we can choose to develop those skills further if we wanted to.
In an early stage startup environment, I think you need to be - and need to have - people who are a "jack of all trades". People who have already done something once to have gone through the procrastination and uncertainty in enough different areas be able to work quicker when working on the startup. You need people who are flexible enough to be able to number crunch the finances, while also developing a marketing plan. They also need to be able to do the most mundane things, such as empty out the office garbage -- because no one else will.
My claim is that starting a business while at business school is a great time to do so. The breadth of courses, extracurriculars and other activities ensure you are always thrown in that place of uncertainty and doubt -- you are always learning how to do new things. You become better prepared to do all these things for when it comes to applying these skills on your startup. While at school, you also certainly can't afford to have anyone take out the garbage than yourself.