Make sure your students are keeping the weight on the ball of the foot and not too high on their releve. Also, the releve stays at the same height the entire time, be careful not increase the releve as you bend the knees. Keeping your heels actively squeezing is important for adductor connection. When you hear of someone complaining about knee pain during plies, it often comes from lack of adductor activation, this why is super important that the heels are resisting each other. The pulsing of the Plies come from the entire thigh not the knees and it is important to keep the angle of the thighs diagonal to the corners of the room and not force the legs to an extreme external rotation. Spine is in neutral position throughout, when the hand is on the hip this will aide in stabilizing the pelvis.
If you add a Passe, make sure the weight is being transferred to the supporting leg completely and keep that leg lengthened and supported. Once again keep your foot stiil, do not change the height of the releve.
Divide this into two steps at the beginning. Go down to your plie, then increase the degree of your plie a little past your comfort zone, but still being able to maintain neutral spine and actively squeezing the heels together. If you go all the way down the muscular engagement is lost and too much pressure is placed on your knee joint on the way back up.