The shoulder blades must be able to both glide along, and settle on, the ribcage. The thoracic spine and ribcage are super important for healthy movement. Feel the scapula being able to move freely and also stabilize. In normal movement and posture, we want the scapula to be sitting comfortably just about an inch away from the spine - not far away from midline, which happens if you hunch forward.
Let shoulder blades glide up, especially with overhead movement. You are not doing your scapula any favors by sinking into your shoulders. Modify as much as you need to, but also, hold the bar high for yourself and retrain your movement patterns. Yoga can make you feel like you are supposed to just sink into the structures of your body. It is really important to do everything from a place of strength and readiness. If this class doesn’t feel hard - then you aren't working all your muscles. Make sure everything, including your glutes, are engaged.
Your body is a hydraulic, energetic system when it is aligned and balanced. These small shifts will create incredible shifts in the way you move and carry yourself. Let’s take that sense and intention to bring more movement and joy into our life and then share it with all beings everywhere.