I think it would be better if you could do the stretching/flex and strengthening as 2 different classes if you can. 10 or 20 minutes of stretching is definitely better than nothing, but it's really not enough to get big flexibility gains. If you must do them both in one class, then DEFINITELY SAVE THE STRETCHING FOR THE END! Do it for the last 15-20 minutes of the class.. muscles stretch better at the end of a workout when the body is really warm, plus it'll save you from having to do a separate cool down (because you'd have to stretch again at the end anyway!).
I teach a pole strengthening class & here's what i do:
10 min warmup: 5 min of jogging around the room, (occassionally making them do the jog where you bring your knees really high) , & zig-zagging around the poles. Sometimes we also do some jumping jacks or jog in place while doing shoulder/arm rolls. Then 5 min of any aerobic "toy", such as hoola hoop, jump rope, etc. I got these wierd things from a toy store.. there's a wheel with a stick that has a loop that you put your foot through, & swing your leg to get it rolling while having to jump over the wheel everytime it comes around your other foot. Hard to explain, but i bet you could find some cool stuff at a big toy store!
Lower body strengthening: 15 min. Leg swings while holding on the pole, both front & side.Lunges going across the room, & then doing 8 squats each time we reach the walls. We do both front as well as diagonal lunges.
Upper body strenghening: 20 min. Tuck lifts, middle lifts (hands in prayer pose hold, lift legs behind you like in carousel spin). Alternating crucifix lifts (lift into crucifix alternating legs & which hand is on top- 20 times so that each side gets 10 reps). They don't have to let go of their hands for this, since the point of the exercise is strengthening, and we want to do it fast & continuously. Rotator cuff/shoulder exercises with wide elastic bands tied on pole. Forearm exercises with bands. Shoulder mount prances for advanced students, or a similar exercise while laying on the floor (on back) for less advanced students.
Then 15 min of stretching.
I hope this gave you some ideas.. good luck!! :))