Graphing Calculators. If your teacher allows the use of phones or tablets on tests, you can use a graphing calculator app instead of an expensive graphing calculator.
Flash Cards. Apps that let you create your own flash cards are useful study tools for many subjects.
Homework Planners. Yikes! How do keep track of all your pending due dates, exams and class schedule? With a homework planner. Look for one that offers alerts for upcoming deadlines. Some will even automatically silence your phone during classes.
Subject Specific Apps. Subjects as diverse as chemistry, Shakespeare, geography, literature and biology (just to name a few) all have interactive study guides in the app store. Just search for your subjects and see what you discover.