5 Growing Ed Tech Companies That Make the GradeDespite some flame-outs, these entrepreneurs are turning solid education technology into big business.

Ed tech is amultibillion-dollar marketthat has drawnmajor investors, sparked itsown news outletsand triggered anavalanche of new ventures. But, as often happens within newer, big-investment spaces, some in and around the industry are predicting a coming bubble burst.

It's true more than a few ed tech companies have failed. Many more will follow. There's no way to know if this is a bubble or just a natural thinning-out of a crowded market. Still, increasingly innovative ed tech companies continue to enter the space. Even in these competitive conditions, some have found profitable models. They're starting to see real gains and seem poised to thrive.

Here are five ed tech companies that are doing more than raising money -- they're making it. Each has crossed the elusive threshold of seven-figure revenue, and a few have scored exponentially more.

Apps let you study on your smartphone.

HLTfound its market niche by producing study guides and other academic material for smartphones. The company already has theNo. 1 study appfor nursing exams, with more than 1 million downloads. HLT now is breaking into other big, potentially lucrative learning products such as real estate and the globally popular "For Dummies" book series.

"Accessibility is the key to successful ed tech," says cofounder Adam Kuene. "People will always need to learn, and we've found that leveraging mobile technology to make learning more convenient and more flexible is great for students and good business."

Related:5 Fascinating Ed Tech Companies from National Education Week

Learning gets gamified for coding and foreign languages.

Tynkermakes game-style apps and educational courses that help young students learn the basics of computer coding. More than 32 million children and 50,000 schools already use Tynker's products. It's not hard to see why: Tynker cracked a fun, important market that gives kids, parents and schools valuable tools to bridge the tech skills gap through STEM education.

Duolingois another gamification learning app, this time for learning another language. The company's apps are in the hands of more than 120 million users around the world. Duolingo was named the free iPhone App of the Year in 2013 and TechCrunch's best ed tech startup of 2014.

Related:5 Challenges Facing Education Tech Startups

Schools adopt digital learning and testing.

DigiExam, an award-winning Swedish company, has developed digital exam platforms for schools. Teachers save time and schools save money through secure, in-class testing. The company counts more than 1,100 schools in greater than 50 countries. DigiExam is planting its flag in the U.S. market as it builds on a customer base that includes Columbia University and other Ivy League-level institutions.

"We're always surprised how many teachers and schools still use paper and pen to give and grade tests," says CEO Johan Hägglund. "If there's one thing that tech is making obsolete, it's paper -- and we're pushing that transformation in ways that benefit teachers, students and schools."

Cleveris winning its share market by making a product that simplifies the chaotic and fluid tech ed marketplace for teachers and schools. Its product allows schools and districts to see and manage hundreds of learning apps in one location. The need is so great that 87 of the top 100 school districts have already signed on.

Related:4 Ways Technology is Making Education More Affordable and Available

The ed tech sector is so young that most of the most promising companies aren't public -- yet. But these successful new companies are proof there's money to be made where technology and education intersect. Companies that endure have the potential to improve the quality and accessibility of education while generating profits for entrepreneurs and investors. Making money by doing good is a win for everyone.