Most dancers have a reasonable amount of flexibility due to the nature of dance, however it is important to think about timing, the type of stretching and how much to do. Stretching is ingrained in each dancer's life in order to improve flexibility, whether it comes naturally or not. Some dancers can become frustrated if their stretching efforts don't pay off, but there are some tricks to help the stretching along a little.
There is lots of research which has been done to show that the timing of stretching is vital. Prolonged and excessive stretching before a dance class or performance can be detrimental to muscle strength and power, and can even increase the risk of injury. Make sure your students do or are taken through a thorough warm up before they do any stretching: stretching the muscles when they are cold may mean they won't 'fire' properly when they are needed. Ideally, stretch after class in warm up layers to increase flexibility, when the muscles are completely warm.
How dancers stretch also has an impact: many dancers focus on passive stretches, where the limbs are held in one position and gravity is used to increase the range of motion. This type of stretch is less likely to achieve true increases in muscles length and may even stress the joints and ligaments. Contracting the muscle against resistance when taken the point to where a strain is felt is more beneficial, as the limb can then move further into the stretch. Ballistic stretching (bouncing in and out of the position) is also not recommended.
As well as stretching it's important to cool down properly after classes, which also decreases the risk of injury and improve muscle recovery. Keep the muscles warm after students stop, to ensure they don't get stiff and the muscles cool down gradually.