Hello Mary Jansen,
Certification, as you know, is the minimum education required to show proof of competence in a field; therefore, I suggest a Group Fitness Instructor cert for “the best training for step aerobics teachers.” I am grateful for my group fitness cert.
I understand your feeling; but, ultimately, you may need another cert if you want to do the best job for your clients. The group certification course will address step class as well as many other options. As always, experience, coupled with your education, is the best teacher.
Here are two links for you to start searching:
Natalie aka NAPS 2 B Fit.