You had been ordered to train with Cassian, you had no choice but to be sent to stay with him. But you did have a choice about whether or not to participate. As much as it pissed Cassian off, he couldn't force you to move your body and partake in the training.

You're sat in the training yard right now, arms and legs crossed, refusing to work. You can see his frustration rising, but you just stare at him unbothered, refusing to move.