"I realised the burden of what teachers go through here when I was living in this rural village," says Toni Maraviglia, co-founder of Eneza Education, a mobile phone based education tool in Kenya.
"If I was giving individual feedback on a daily basis as a teacher I would see my kids scores just skyrocket, but in some classrooms there might be 70, 80 students, and you can imagine how overwhelming that is for one teacher."
From her experience, Ms Maraviglia, a 32-year-old American who had moved to Kenya to help start a program to support teachers, came up with the idea of teaching via mobile phone text messages.
Together with co-founder Kago Kagichiri they set up their business in Nairobi in 2011.
Eneza says it wants to make "50 million students in Africa smarter".
"Eneza is a virtual tutor and teachers' assistant," explains Ms Maraviglia. "It's a way for students to access courses through a low-cost cell phone.
"A lot of people [wrongly] think that our company is non-profit, because I am a woman, and because it's in education.
"We are a mission-based, for-profit company - what we do is charge an extremely low cost to our users and our business model is based on large scale."
Dialling up change
For many young people across Africa, the education available to them is often both under-resourced and costly to students and their families.
Limited public funds leave classrooms over-crowded, teachers over-stretched, and textbooks in short supply, while private schooling is prohibitively expensive for many.
Ms Maraviglia, who started out as a teacher in inner city New York after graduating from the University of California, jokes that she is the teacher who hated technology, and her Kenyan co-founder Mr Kagichiri is the techie who hated teachers.
"What really changed my mind about what's possible is Kago," she says. "When a technologist and a teacher put their heads together they are able to really think of solutions that work."
Eneza's 500,000 users access courses and quizzes almost exclusively by text messages, for a cost of just 10 Kenyan shillings (10 cents; six pence) per week, which is deducted from pre-paid airtime on their mobile phones. The company has also developed Android and web-based versions of the app, but text messages remain the vast majority of its operations.
The emphasis on text messages enables users across rural Kenya to continue learning even if people can't afford to pay for data or expensive handsets, or if there is no data network in the area.
Eneza's courses target students from around 10 to 18-years-old, but also cater for school drop-outs up to the age of 25.
They are aligned to Kenya's national curriculum and cover subjects including maths, science, Kiswahili and English.
Learners receive questions - both multiple choice and open-ended - by text, and get feedback on their responses, whether right or wrong.
The app also features a built-in detective game called "Where is Ms Mandizi?" that sends more clues as learners progress, and users also accumulate points as they work their way through lessons.
"We are trying to bring 'gamefication' into the app, along with access to Wikipedia and an interactive chat - an environment in which people are learning, but at the same time being incentivised by fun things," says Mr Kagichiri, who taught himself to programme when he was eight-years-old.
Of the 500,000 users in Kenya - where mobile phone penetration is at one of the highest levels in sub-Saharan Africa - 70,000 are active users on a monthly basis.
"What we see from our students is that many of them will go home and study on this from 20 minutes to an hour every night," explains Ms Maraviglia.
Some of Eneza's so-called super-users are located in very remote parts of the country, including in Dadaab refugee camp near the Kenyan border with Somalia.
And while the Eneza team believes that at least 30% of their users do not go to school, in that they are drop-outs or older learners, the traditional education system remains critical to their work.
"From just 10 shillings every week, my child is given so many questions a day," says Beatrice Wambui, the mother of 12-year-old Peninah who is a heavy user of Eneza.
"There were some topics my child did not know, Eneza stepped in and helped her.
"After she started using Eneza, she moved to second position at school, she had been in seventh place before."
The team hope to roll out the Eneza app, "Eneza" means "to spread" in Swahili, in other African countries and they have already begun work in Tanzania and Ghana.
"We are really looking to be huge," says Ms Maraviglia.
"We want to be the go-to mobile educational source for Africa, and 50 million students is the target we want to hit in terms of reach."
The Digital Disruptors is a series about the people and companies shaking up business with new technology.