There are what are called the 8 Limbs of Yoga. Study of all are necessary if one wishes to reach enlightenment. One of those Limbs, called the Asanas or Postures, are used to prepare the body to sit in meditation.
A class that concentrates on a watered-down group-taught version of the Postures are not an introduction to religion.
If anything, it could be seen as an introduction to self-control.