Here is a circuit I did with my class the other day that went off!
You need to set up a station for each person, it should include;
Each person stays at their station and the class does the same move at the same time;
We went through the circuit 4 times, the first time we did each exercise for 2 minutes, second and third times for 1 minute at each exercise and the forth time 30 seconds for each exercise. I call out the next exercise at the end of the time. I took the class for a run/power walk after the second round before returning to complete the third and forth rounds.
Keeping changes quick, weights are optinal for things like squats, athletic lunges and I tried to vary the exercise each time through to keep it fresh, like doing squats with one foot on the step, backward stepping lunges and side to side push ups etc.
Need any further explanation give me a yell and I'll try and explain.