I’d send out a group email to all of your students, mentioning in broad terms that you’d just like to make them aware of a new cancellation policy that you’re putting in place. Say broad things like: “Bit of a change of pace here at Singing Lessons Exeter. I didn’t want to catch anyone out so I thought I’d let you all know: it’s industry practice to have a same-day cancellation policy. So if you pop me an email the night before, that’s fine and dandy. But if you cancel the same day as the lesson, the fee will still be due. Unless I manage to sneak someone in to fill your place, of course.”
(Please do steal that word-for-word :D)