(CNN) -- It's a Tuesday morning in January, and seventh-grader Katerina Christhilf is learning algebra. But it's no ordinary class. This one takes place entirely online, led by a teacher a few miles away.

As part of her training to become a ballerina, Katerina takes dance lessons four times a week, including up to eight hours on Fridays. All that training makes it hard to go to a conventional school, so she takes science, literature, composition, vocabulary, history, music, art and French - a full course-load - from the comfort of her home, through Virginia Virtual Academy, a program run by K12 Inc. that began operating in the state in 2009.

"Ballet is really important to me and it's usually in the mornings, so if I went to school I would only be able to go on the weekends," Katerina explained. "Sometimes I'll study in the morning and I'll do a few classes and then I'll go to ballet for maybe like three or four hours and I'll come back home and I'll do some more."

Katerina is one of a growing number of students who go to school online full time. About a quarter of a million students in kindergarten through 12th grade were enrolled in full-time online schools last year, according to the International Association for K-12 Online Learning, a 25% increase over the previous year. Some parents choose these schools because their children are struggling in traditional schools; others do so for their flexible schedules.

But as the number of students learning online full time has grown, so have questions about the effectiveness of that approach.

"There's so much more to learning than content acquisition," said Dennis Van Roekel, president of the National Education Association, a teachers union that supports the use of some online content to enhance learning in brick-and-mortar schools. "There's socialization. There's discussion in classroom. And as a teacher for 23 years, you know there are times when they just need a little encouragement. You've got to be able to look at their face and know whether they need a push or maybe a hug."

K12 tries to incorporate interaction into its curriculum. Katerina listens to her teacher's instructions through a microphone and follows along with the on-screen lesson, clicking from page to page and solving math problems. She can ask questions by typing into a chat box or using her microphone, and she uses emoticons like a smiley face or a confused face, and thumbs up and thumbs down buttons, to indicate whether she understands the concepts being taught.

"We always try to keep them active," said Katerina's teacher, Jessica Henry, who has taught math for 12 years, the last two of them online. Henry said she sometimes pauses a class to call a student at home if they are not participating at the level expected. "In this environment we also have to do family conferences once every month for half an hour. So not only do I know the student, I know the parents well. They're comfortable contacting me through e-mail and they contact me all the time."

Students also send in monthly work samples that are graded, and take frequent tests, called "assessments," to track their learning. But not all of Katerina's work is done online. K12 ships additional materials, like textbooks and worksheets, to students to be used offline. And Katerina takes part in field trips about once a month to spend time with other students and with her teachers.

K12 is one of the nation's largest online educators. Founded in 1999, the company offers full-time online public school programs for at least some portion of the kindergarten through 12th grade population in 28 states. The company contracts with school districts that use state aid to offer public school students who enroll through their districts a free full-time online education - though out of district registration fees sometimes apply. K12 also sells products and services, including individual courses, to more than 2,000 school districts nationwide, all of which helped it bring in some $522 million in revenues in fiscal year 2011, nearly 36% more than in the previous year.

"School districts want this, teachers want this and students want this," said K12 Founder and CEO Ron Packard. "Five years ago we were about one-fifth the size we are now . And I would expect, we've grown fivefold over that period, that we would probably fivefold again over the next, you know, five to ten years. This is a movement that's just beginning, so we're in the first few innings of a long game."

A matter of debate

Packard describes technology-based learning as powerful and believes that as technology becomes more ubiquitous, virtual schooling will make more and more sense. Still, critics of full-time virtual schools worry about the quality of education that companies like K12 deliver.

A U.S. Department of Education report published in September 2010 found that more study was needed to determine the effectiveness of online education for kindergarten through 12th grade students.

Data from the Virginia Department of Education showed that, as a group, students at Virginia Virtual Academy did not perform as well on state reading, math, science and history tests as their counterparts at brick-and mortar-schools in Carroll County, where K12 has a contract. Packard said many of the academy's 400 students were behind grade level when they joined and that K12 data shows performance improves the longer a student is enrolled.

"You've got to measure where they start from, right? And that's the big question. So the question is, are they learning at a higher rate in a virtual school than they did previously?" he said. "That school is new, so the students couldn't have been there all that long before they took the test."

Using taxpayer dollars to fund online public education is also a concern for those worried about a public school funding system that is already strapped in many places, especially as states facing budget shortfalls cut back.

"School funding is in such dire straits right now," said Van Roekel. "If they take that money out to enhance some - a few - or private enterprise because 'I sell good online curriculum,' that's a mistake for students. They suffer."

Supporters and critics of virtual schools agree that online learning doesn't work for all students and that parental involvement matters. Bright, highly motivated students may thrive, while those who struggled in ordinary schools may continue to struggle online. Parents of K12 Inc. students are expected to act as "learning coaches" to help facilitate their children's learning in the lower grades.

The debate over online education is sure to go on as the companies that provide these services continue to expand. In the meantime, after receiving top scores on state tests in history, reading and math last year, according to her mother, Katerina and her family believe it's the right choice for her.

"I think it should be an option for people who want to do it to be able to do it," said her mother, Elena Christhilf.