Editor's note: ClassDojo is one of Inc.'s 2015 30 Under 30. This year's readers' choice winner is ThinkLite.   

How did ClassDojo co-founders Sam Chaudhary and Liam Don get their classroom management app into the hands of more than 3 million teachers around the globe? One thing they know for sure is that it wasn't because of their marketing efforts. 

They've never had any. 

"It really became a word-of-mouth thing," Don says.

Teachers use ClassDojo to track students' development of what Chaudhary refers to as "soft skills," such as persistence, curiosity, and focus. They can also send private messages to parents, who can log in to a different view of the app to see their child's progress in these areas. A third ClassDojo interface allows students to view their teachers' feedback. 

Listening to users

The app launched in 2011 with 80 teachers who were part of an initial focus group during its development. Four weeks later, 12,000 teachers and students had downloaded the free app.

Don says that ever since, the company's growth strategy has been to improve ClassDojo according to its growing user base's feedback. For example, in addition to being able to send parents individual messages, teachers wanted a way to send one-way group messages to parents on topics such as field trips or classwide projects. This led ClassDojo to build a broadcast feature.

At its heart, Chaudhary says, the app is a communication platform meant to encourage frequent dialogue among teachers, parents, and students.  

Chaudhary, a former high school teacher and McKinsey & Company analyst, says that ClassDojo focuses specifically on monitoring social and emotional learning, rather than on grades, because these are the skills that students will carry with them as they get older. 

"You and I know exactly how much of the stuff we learned in high school we actually use in the work force every day," Chaudhary explains. "You'll realize that it's very little of the actual content, but probably quite a lot of the skills that we developed--which, ironically, we were never really assessed on." 

But not everyone is convinced that tracking behavior in the classroom is beneficial to student development in the long run. Critics such as privacy law scholars and some parents are worried about the potential harmful repercussions of collecting this kind of data. For example, some fear it will lead to certain students unfairly receiving a "problem child" label that will stick with them.

Protecting sensitive data

Don says that ClassDojo has taken measures to avoid unintended consequences. For example, the company deletes student data after one year. 

"No part of what we do requires that data, because we just use it for communications between teachers and parents and students," he says.

ClassDojo, which hasn't yet landed on a business model, has also promised never to advertise on the platform. The company is instead considering a handful of other possible ways to generate revenue, such as enabling parents to pay for premium features. 

Tim Brady, ClassDojo board member and founder of the education-focused accelerator Imagine K12--through which he is an investor--says the decision not to use student data to offer targeted advertising is a reflection of the company's and its founders' high ethical standards. 

"If what you're doing is trying to train people in character, you can't take shortcuts," Brady says. "In many ways it's almost an easy decision, because this is what they're trying to teach or help others teach kids."

Chaudhary and Don came to know Brady during their stint at Imagine K12, which they applied to while still living in the United Kingdom. The two met in 2011 at a startup weekend event in Cambridge just three months prior to deciding to move to Silicon Valley. 

"One of the most amazing things about being out here is the belief that you can create something that will reach significant scale," Don says. 

Corrections and Amplifications: Approximately 3 million teachers use ClassDojo worldwide. An earlier version of this article inaccurately listed how many teachers use the app.