(FORTUNE Magazine) – Here's a business-school case study for class discussion: What happens when all the MBA students ditch school to seek their fortunes on the Web?

That hasn't happened just yet, but as classes reconvened this fall, there were some empty seats. At Northwestern's Kellogg School of Management, 12 students out of 640 didn't return for their second year, compared with just one last year (and that was for health reasons). At the University of Pennsylvania's Wharton School, 25 out of 790 students didn't come back; at least 21 of those fled for the dot.com companies they interned with over the summer. "We've never had this before, never in our history," says Edmund Wilson, Kellogg's dean of students.

These no-shows are stoking a debate about whether MBA programs are losing their relevance in an era of instant wealth creation. "The opportunity cost [of going to business school] has gone through the roof," says 30-year-old Jeff Smith. He was supposed to be in his second year at MIT's Sloan School of Management right now, but instead he's still at AdventureSeek.com, a San Francisco startup where he worked this summer. "The economy is moving so quickly right now," says Smith, "I just don't want to miss the chance to make this company what it is."

His B-school roommate, 26-year-old Jason Salfen, didn't return to MIT either, opting instead to stay at eDentalStore in Burlingame, Calif. He figured the decision would give him lots of options--of both the career and stock varieties. "There will be 45 employees here in a year," Salfen reasons, "and there's a huge difference between being the fifth employee and the 45th." The erstwhile roommates, both of whom plan to finish their degrees eventually, have had to sublet their Boston apartment.

The business schools, on the other hand, can't sublet their vacant seats. Richard Kurovsky, communications director at Berkeley's Haas School of Business, says the key to keeping students on the farm is providing "pressure release valves" in the form of e-commerce classes, work-study programs, business-plan competitions, and the like. Some schools are refashioning themselves as business incubators, touting their access to entrepreneurial expertise and even investors. But for schools like Kellogg and Wharton, which are near no major technology hubs, a few alarm bells are ringing. In late August, a reporter from the student newspaper asked Kellogg's Dean Wilson to verify a rumor that 100 students weren't returning. That rumor, of course, proved wildly overblown. Wilson just hopes it doesn't come true next year.

--Jerry Useem