Apps every student must have: "Khan Academy". Khan Academy is an awesome tool that includes videos on every mathematics topic you can think of, as well as a number of science topics, as well. Khan Academy videos are short videos of how to do specific problems the way your teacher showed you!
Some of the specific apps include: Pre-Algebra, Algebra 1, Chemistry 1, Biology 1, etc. Just go to the app. store and search for "Khan Academy".