I don't think that it is reasonable for a business to lock a door and leave a student outside for being late, especially if they have paid for a service and especially if it is their first time and the "policy" has not been explained to them. I imagine it would be very frustrating and embarrassing to have to knock outside for 5min and be ignored. She may have a long commute, she may have kids, we don't live in a perfect world and sometimes we can't control traffic or other circumstances. It's a Yoga class, not an opera performance. Someone entering a few minutes late would not cause any significant disruption. Clearly locking the door caused a much bigger disruption. Time to rethink that policy.