What is the fastest bpm for step aerobics that is considered to be safe practice?
Ariadne gave a great answer. I would add to that by asking how many risers are being used. The original research with 2 risers and BPM in the low 120s equated the VO2 max of a step class with running. Despite this, many people raised the BPM and lowered the risers. This however lowered the VO2 max, but participants liked it because it felt they were working harder. I have seen some fun choreography that mixes step and floor aerobics. The music is usually faster, but they generally use 1 or no risers. If this makes participants happy it is probably safer than the fast speed on 2 risers.
There is indeed a fine balance between keeping people happy and making sure they are safe. You can educate them that they are burning more calories (makes more sense to the clients) at a lower BPM on 2 risers, than a fast BPM on 1 or no risers.