Garageband is an app you can use to compose, record, edit, and mix. This app also includes built in instruments you can play electronically so you don't have to find the actual instrument to record. This app would be useful in a classroom setting to start teaching students how to compose or mix even if they an't necessarily play the instrument themselves.
Tenuto is an app that has a collection of exercise that you can customize to fit your needs. I would use this app as an exercise for students because it is an introduction to music theory and allows you to enhance your ear training skills.
Nota is a music theory app. I would use this app to quiz students and I would encourage them to use this app at home to practice and stay on top of their theory. This app comes with a piano and teaches and quizzes on chords, scales, time signatures, and more.
Karajan Music and Ear Trainer quizzes you on scale qualities, chord qualities, pitch and tempo. Using this app would enhance ear training skills and could be used both in the classroom and at home for practice.