Problem with Kokutsu Dachi?

The back stance is a difficult one to move out of and requires a lot of hip and leg strength. Many people have to first lean or they might even lurch when they start to move out of it instead of keeping their backs straight and maintaining that 70/30 weight distribution until the last possible second. Start doing some lunges to strengthen your hips and legs and work on this stance and moving out of it and into another one several times back and forth after your class and pay attention to your posture. Also having your stance a half shoulder width wide will help give you better balance rather than one foot lined up directly with the other, heel-to-heel.