Khan Academy is the official app of the prestigious academy of the same name, a free online service where any person in the world can learn about lots of different subjects. Basic math, world history, cosmology, computer science ... you'll find subjects to suit every taste.
This official app will allow you to comfortably access all the online portal's content from an Android device. The only thing you need to do is log in with your user account and then you can start enjoying the app's interface, which is perfectly adapted to touchscreen devices.
In Khan Academy, it doesn't matter if you're a second-grade student or a professor: you'll always find lessons to learn, exercises to do, and interesting videos to watch. In addition, every time you complete a lesson (watch a video, do some exercises), you'll receive points and medals.
Khan Academy is an excellent official app for an excellent service that brings quality education to many users around the world.