The responses given by Janet, Wendy and LaRue are all excellent. The first question that pops into my head is what do you mean by “small group?” 3-4 or 10-12? The number of clients in each session will have a significant impact on the conduct of your session. If you gear it to the most fit, the least fit will quickly burn out. If you gear it to the least fit, the most fit may well become less than interested. So, it’s a challenge to your teaching expertise to make sure that each clients gets a good, effective workout.
The other question is the design of your class. Is it cardiovascular in intent, strength, flexibility or some combination? Depending on your equipment availability you can design a class that will challenge all fitness levels. Consider a circuit class if you have the equipment.