There are many ways to improve your stretching. The first way seems simple, but is not easy to do...you need to stretch every day. Not only should you stretch before and after dance class, but you should perform some sort of stretch routine each day, perhaps upon awakening in the morning or before going to bed at night. With stretching, more is better than less. (Of course, never stretch to the point of pain, and never force a stretch on an injury.)
As for your splits, these really take time. Splits must never be forced, as the muscles in the legs are easily pulled by overstretching. In order to get down to the floor in a split, you have to gain flexibility in your hamstrings as well as your back. Slow, easy stretches seem to be most effective. Never plop right down into a split position...ease your way down, lowering yourself slowly without bouncing. Concentrate on relaxing the area you are stretching, push into it slowly and gently.
A great time to stretch is after class, after you are warmed up. Muscles are most receptive to stretching after they are warmed up. Use this time to your advantage. You'll be surprised how much it will help your flexibility.
Finally, try not to compare your flexibility to that of other dancers. Some people seem to be naturally more flexible than others, without as much effort. With a little self-discipline, however, you too will have your splits. Set small goals for your flexibility and reward your efforts.