I recommend a variety of apps to aid study (and teaching). Websites like tech4teachers.com are wonderful for discovering new apps. Those below, however, are the ones I personally recommend for now:
Sleep as Android: Useful for being fit and healthy. Alarms at smart times. Adjusts for weekends. Helps avoid oversleeping and undersleeping when going to bed later. For those who dislike being told when to go to bed (and I know a few!) it can simply track how much you are sleeping. For teenagers, though, I feel it is essential:
Quizlet: Create own quizzes. Share quizzes. Develop quote retention. Tremendous when students use this in class as they work through texts.
Evernote: Create online study groups. Both receive and create content. I prefer this for sharing resources over the ability to store on Moodle or similar VLEs. From this, I also believe that it is effective for recording student work, and for students sharing their work, too. If I could guide all my students to create Evernote accounts, I would.