Think about the kids who struggle the most in your bands, choirs, orchestras, athletic programs, dance teams, etc.
So many of my students that struggle to articulate clearly, play with nasal sounds, can't breath deeply, run out of air quickly, and generally don't have good coordination are also mouth-breathers.
While awake, sleeping, or both, they breathe through their mouth instead of their nose. These students tend to remain undetected because we breathe through our mouths in the band world to take in larger amounts of air than the nostrils can naturally provide.
Mouth-breathing is only okay when playing a wind instrument and if you have a cold or flu and literally cannot breathe through your nose. It's SUPPOSED to be an alternate to nostril-breathing to stay alive. It shouldn't be how we breathe all the time.
Mouth-breathing can be linked to, or worsen, a whole host of conditions and diseases like sleep apnea, high blood pressure, ADHD, asthma, chronic sinus infections...just to name a few.
The mouth-breathers I've come into contact with frequently misuse their tongue and have decreased sensitivity inside their mouth, so they can't feel their tongue well or whether or not they are using it correctly.
Mouth-breathing also changes the shape of our sinus cavities and our teeth/jaw/skull. These students struggle with allowing their facial mask to vibrate and resonate because the sinuses close when not being used to breathe.
What can we do to help our students? Ask them to gently bring their lips together and breathe through their nose throughout the day. They will never become the musician/performer they can be if they mouth-breathe. And they will have health problems for their entire lives.
Book Recommendation: Breath by James Nestor. Absolutely wonderful read that humorously enumerates and describes the ill-affects of mouth-breathing, how learning to nostril-breathe positively changed his life, and a wide variety of breathing exercises in the epilogue.