HappyKey

HappyKey provides you with a simple keyboard with many useful math symbols. It will fulfill all of your math needs with numbers, superscripts, pi, math operators and relations, infinity, and beyond.
HappyKey works with Happy Grader (happygrader.com), allowing you to be even happier when grading exams. Happy Grader is a PC application that helps teachers grade happy by automating the grading process while utilizing Google Forms and Microsoft Excel. Students can enter answers to their math tests with HappyKey, and teachers can be sure that there will be continuity across answers, since all students will be using the same keyboard characters.
Install the app first, then to setup the keyboard go to Settings > General > Keyboard > Keyboards > Add New Keyboard > HappyKey
Credit for the keyboard functionality and design goes to the open source Tasty Imitation Keyboard by Alexei Baboulevitch (GitHub user archagon).