Michael Dell enrolled as a biology major at University of Texas but spent more time fiddling with stacks of computer parts in his dorm room than hitting up the library. Instead of studying, he started selling new computers through advertisements in local papers.

It was a lucrative distraction. By the end of his freshman year, Dell was selling about $80,000 a month in computers. With the money rolling in, Dell decided not to return to school.

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He dropped out of college at 19 to run the company that would become Dell Inc. Within the next few years, Dell's annual sales passed $100 million. This March, Forbes' pegged Michael Dell's net worth at $16.4 billion.

Our most recent list of the world's richest included 1,125 billionaires. At least 73 of them, like Dell, dropped out of some stage of schooling.

Those 73 are like Dell in another way too: They didn't drop out to watch daytime television on the couch. They left school to work hard.

Dell explained his attitude to University of Texas grads at a 2003 commencement address: "Circle the pitfalls and highlight the opportunities. Then build a vision of how it could all be better and work like hell to make it happen."

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Sheldon Adelson is another billionaire lacking a degree but possessing plenty of hustle. Adelson enrolled at City College of New York but didn't finish, probably because he was too busy with other ventures.

When he was 12, Adelson borrowed $200 from his uncle to start selling newspapers. He dropped out of college to become a court reporter. He also worked as an ad salesman, a consultant, and a tour-business operator.

That relentless drive led him to his first big windfall. He organized the computer industry trade show Comdex and made handsome profits leasing out exhibition space. He's since jumped into casinos, where he's been adding to his fortune ever since. In Forbes' most recent list of the world's billionaires, he ranked 12th with a net worth of $26 billion.

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Some billionaires didn't even make it as far as Adelson in school. Richard Branson, who had dyslexia, was a lousy student. He dropped out at 16 to start a magazine.

To fund the publication, he also started a mail-order record business; the venture grew into Virgin Records. He took a risk by signing a raucous band called the Sex Pistols, which had already been cut from two other labels. Other hit acts followed, including Boy George and Peter Gabriel.

Plenty of other companies have followed as well. He's since expanded into airlines, health insurance and medical care. Next stop: space. His latest company is Virgin Galactic, which hopes to shoot tourists beyond the Earth's atmosphere.

But don't get the idea from these billionaire dropouts that school is worthless. Even the world's most famous dropout (and its third-richest man) acknowledges the importance of a good education.

Bill Gates left Harvard during his junior year to work on a little company he'd started called Microsoft. He recently testified before Congress on the importance of improving the US education system.

"Too many of our students fail to graduate from high school with the basic skills they will need to succeed in the 21st-century economy, much less prepared for the rigors of college and career," said Gates.

School has other benefits too, like who you meet. In 2000, billionaire Steve Ballmer took over the role of chief executive of Microsoft from Gates. The two lived down the hall from each other while they were both students at Harvard.