Such policies will differ depending on your business model. What I mean is, do you allow drop ins, how much space do you have, (if you charge to cancel a reservation, but allow drop ins and your classes are not full, people will not be motivated to sign up in advance).
In the stand alone studios I have had an association with the policy has generally been that one could sign up in advance, but if one were going to cancel in less than 24 hours one would be charged.
I seem to remember one place that would not charge to cancel within 24 hours if there was a student on waitlist who took the slot.
I also know a teacher who would not charge twice…. on the idea that stuff happens to everyone, but that if you use up your forgiveness when it is not an emergency you understand it is gone.
When I had a large space I just set my times, and let people drop in, as that seemed easier, but that was well before online registration which has made things so much simpler.
I would suggest visiting a few studios in your location and seeing what the general policy is in your location. If people are used to a way of doing things it can make it easier to structure it that way.