Are you holding the tights up with a belt? Dance tights have excess fabric in the waist so you can pull them up as high as they'll go, then wrap them over a web or elastic belt 2 or 3 times, bringing them back to your natural waist.
Are the legs long enough? You are at the large end of the size grid for wearing that size tights. If the legs are too short, they'll pull the crotch down and make it sag. It seems contradictory to upsize something the is skintight, but sometimes that works the best when you use the belt-wrap technique to get rid of extra fabric. [/i]