Ahhh the barre stretch... one of the most relaxing, beneficial, and for some, intimidating parts of class.
As always, we encourage all of our students to listen to their body. Does your leg on the barre seem like a bit too much? Feel free to bring your heel down to your mat or even use the lower window-sill for a less intense stretch.
A few other things to think about:
Alignment: Focus on proper alignment of the hips, shoulder and feet by making sure you are listening for your instructor's cues. In particular, make sure you are bringing your supporting foot with you as you pivot into a new position to avoid any twisting though the knee.