If you consider yourself as a school, you should have at least a instructor training., which includes the chance to get certified. It's not only the hard-skills you improve in one of these courses. Candidates get a lot of new tools how to teach and how to run a course in a safe and successful way. The benefit is maybe not the certificate, it's the whole process of certification. Is all about getting new tools, sharing knowledge and experience.
Most of my students consider themselves as good skilled paddlers when they show up. And even if they have already great skills, they leave the course with sharper tools an most likely with some new. That's my experience as Instructor trainer.
In my opinion, it's a sign of quality to have skilled persons in a rental station. The invest ist low and the benefit for employees, customers and SUP in general is great!