Take a Breath
Read Aadil's reply:
Convictions that come from knowledge and experience cannot be swayed by mere opinions. After more than four decades of practice, it is clear to me that asana practice should never be done while breathing through the mouth. Both the inhalation and exhalation must be done through the nose.
Not only does this stimulate brain activity and focus the mind, but speaking purely from a physiological standpoint, this accomplishes a number of things simultaneously: It moistens the air, thereby preventing drying of the lungs; it warms the air, thereby preventing lung diseases such as asthma and bronchitis; and it filters the air, thereby preventing particulate matter from reaching the lungs. Seen in this light, breathing from the mouth is unsafe.
Yogically speaking, prana is not absorbed from the air during mouth breathing. Prana can only be absorbed through the channels of the sinuses during nasal breathing.
So, aside from finding more knowledgeable substitutes for your classes, I would suggest educating them about breathing before your students get confused and perhaps even injured. You might like to share with your substitute teachers what my teacher B.K.S. Iyengar often would say to us: "The nose is made for breathing, the mouth is made for eating. So, if you breathe through the mouth, I will have to push food up through your nose!"