Appointment Tool

Hi There i need help or a recommendation for wich Calender tool would fit my needs the most… Apparently Calendly isn’t the right one for me, as i already contacted them… It needs to be multi-day or multi-month event compatible.

The thing is i have a Yoga-Teacher Client and she want to have a store on her website for Costumers to buy a 3 Month module. The monthly module is always for example from Okt.-Dec. and then from Jan.-Mar. So in this 3 month the Costumer can come every Thursday from 9:00-10:30.
How would you solve this so she still can keep up with how many people are in her classes? (for example max. 15)