Is undergrad business school useful for running a startup?

Not snarky but, as opposed to what? No school? An arts degree? Or are you looking for an alternative to an undergrad biz school degree that's more start up focused?

Plenty of school now seem to have entrepreneurial programs. Maybe a undergrad school that has an entrepreneurial emphasis is your best bet?

Two ways to make money, financial speculation or selling to other people. Most prefer the latter -- less of a chance of going broke.

To the extent that any business school (undergrad or later) introduces you to other people, that helps.