A pre-class warm-up and stretch is important for increasing and maintaining flexibility. Ballet technique requires long, lean muscles...these muscles can become tight if they are not properly stretched. Even though most ballet classes begin with a long, slow routine at the barre, stretching a bit beforehand is important for preventing tiny muscle strains and small tears that could keep you from improving your technique later in the class.
Do your body a favor by allowing time for a proper warm up and stretch before class. A few minutes will make a significant difference in the quality of your dancing.