Cathy Kramer of Muscatine, Iowa, loved her job in marketing, but at 51 she still dreamed of becoming a classroom teacher.

"I was gone a lot, and travel was getting a little old," she recalled. "My kids were 13 and 11, and I was missing them growing up. And I always wanted to teach."

To get her master's degree in education, she enrolled at the third-largest university in Iowa based on full- and part-time enrollment — and it wasn't the University of Northern Iowa. Now, Kramer is a science instructor at Muscatine High School — and she did most of her class work sitting in front of her home computer in her bunny slippers.

Kramer was a member of the first Iowa group of career-changing teacher graduates of Kaplan University, a rapidly growing, for-profit institution where most classes are taught online.

Its national headquarters are in a Davenport strip mall, with additional offices and support staff scattered from Chicago to Florida. Kaplan has no football team or dorms or frat parties, but it's emerging as one of Iowa's largest institutions of higher learning.

Kaplan, a Washington Post Co. property planted in the prairie, has quietly mushroomed from an enrollment of about 1,500 five years ago to more than 23,000 students today from across the country. The school's enrollment has soared as online schools have struggled to gain credibility in the world of higher education.

Kaplan and similar online institutions don't usually attract the typical 18-year-old high school graduate. Education experts say they are tapping into a market of adult students itching to change their lives and improve their salaries, and willing to pay a minimum of $320 per credit hour to do it.

"They value the flexibility," said David Clinefelter, provost of Kaplan University.

The average age is 34, and most students hold down full-time jobs and have families at the same time, he said.

Most of Kaplan's students are not Iowans, despite the college's roots tracing back to the founding of a business school in Davenport 70 years ago. Students are recruited by advertisements on the Internet or television.

"There was no Kaplan University 10 years ago. It really has been created out of nothing," said Richard Garrett, an analyst of online higher education for Eduventures, a Boston education research firm.

Nearly 3.2 million students took at least one online course in fall 2005, up from 2.3 million the previous year, according to a 2006 report by the Babson Survey Research Group. More than 80 percent were undergraduates.

Traditional brick-and-mortar universities have some online offerings, but for the most part they struggle to serve adult students well, said David Breneman, dean of the Curry School of Education at the University of Virginia.

Some Kaplan students have tried traditional colleges. Michelle McCoy, 26, attended Iowa State University in Ames for a while, but didn't care for it much, blanching at a 600-student lecture hall.

After earning a bachelor's degree from Hamilton College in Des Moines, a traditional for-profit college that is part of Kaplan, she then opted to work on her master's degree in business at night — completely online and from Kaplan.

"It's different," she said. "You definitely have to be a self-starter. ... It was very difficult at first. I felt overwhelmed."

Now, though, she said she's learning more than she did in a classroom, in part because an online student can't hide in the back row.

Kaplan officials tout their programs' quality, the responsiveness of professors — many of them moonlighting professionals — and the attention paid to curriculum. The university is accredited by the North Central Association of Colleges and Schools. Curriculum content is standardized, which makes it easier to measure outcomes, said Clinefelter.