Here is a demonstration of the Back-Saver Sit and Reach test, and is one of the many tests used by the School Fitnessgram Program. The back saver sit and reach flexibility test is different from the traditional sit and reach test as it assesses one leg at a time, rather than both legs straight.
A demonstration of a few of the incorrect ways of doing the sit and reach test. You must hold the position for two seconds, no jerky movements and the knees have to remain straight. In this video the knees are held down using a strap, which is not commonly used but seems effective.
Here is a clear demo of the sit and reach test. Notice that he does not remove his shoes. This test is usually performed with shoes off, as the thickness of the base of the shoes will affect the score. Also, this sit and reach box has the zero mark at the level of the feet, which not all of them have.