An easy way to keep your guitar in tune is with a Tuning App.
I recommend this Tuner App. It's very simple to use.
I also recommend this app to help keep students in time. Metronome Ϟ ONYX Apps
Many songs have the Tempo or BMP indicated at the start of the music. This will give pupils a good indication of the correct tempo of the piece.
I recommend the Jose Ferrer Student 3/4 size guitars and the Admira full size guitars, please get in touch for further advice