For backing tracks I use Youtube, that way the student can then use the exact same track that we have practiced with in the lesson! I also record myself playing piano in the lesson if I am accompanying or having to transpose to a different key just using my iPad.
A brilliant app I have found – it’s called Virtuoso and it’s free, it’s a ‘piano’ on your phone/iPad and you can practice scales/arpeggios and sing along, so really good for warm ups