That 50 point drop early on is fairly normal when going from rest to activity. Blood glucose is the most readily available form of fuel your body has, so it immediately pulls that from your blood before the other fuel sources - lipolysis and muscle glycogen - are put into play. That is what causes the early drop.
You can help reduce that early drop by doing two things. Take a few carbs right before you start, and turn off your basal a little earlier than 10 minutes before.
The spike afterward might be from either the juice carbs, or it might be from exercise intensity. If you are doing a hard finish and your heart rate rises to a certain level of intensity, your body will release cortisol which will cause your liver to release fuel for the energy that you are using. It is very much like the fight or flight response.
If you are spiking predictably, you can turn your pump back on a little before the class is over, or just bolus a little before the intense section of the exercise.
Possibly, reducing that early drop and not needing as much juice in the middle of the class will also help reduce the spike that happens afterward.