You could give her a call and say "I was finalizing the class preparations and headcount for Saturday's class and noticed that you have not signed up (paid your fee) for the class as yet. Do you still plan on attending? If so, I will need you to signup and pay prior to the class."
She shouldn't but excuses of she argues: You need to make special seating so she can sit next to friend, more food/snacks (if you supply), parking... And the main one: You don't let non-paying into class anymore because of seating, not fair to paying students, etc. AND the Disneyland reason (love that one).