Yes, possibly, at least the fit ones. You must warn your swimmers about pressure on the ears and the danger of hyperventilation. Monitor them while they swim underwater, otherwise you run the risk of suffering irreversible consequences.
Although the dangers of breath holding have been known for many years, education efforts have been spotty.
Training materials from the American Red Cross and the YMCA mention the dangers of breath holding, but many facilities still lack the appropriate sign posts.
Among the public there is very little awareness of the dangers of hyperventilation before breath holding.
Also be mindful of your ears. Balance out the pressure. Don't swim with a blocked nose.
Only do this under lifeguard supervision. Make sure your lifeguards know what you are doing and are ready to assist. They should dress warm, as they may not move about in the water as much as you do.
If the water is cold, get wet slowly to avoid sudden shock. The first few minutes might be a bit of a challenge as the water seeps in and soaks your clothes, but soon the water inside your clothes warms up a little.
It is very hard to do anything underwater while holding your nose. You can't do handstands correctly, flip easily, or do competitive swimming strokes.
Your body has certain, natural reactions to water that take time to adapt to correctly. Breathing so that water doesn't go up your nose will happen, but it may take a while.
Practice blowing air through your nose slower, and slower. Until, instead of a steady stream of bubbles you have just enough air pressure in your nose to prevent water from going in. Be patient.
Dress up warm in sportswear or casual clothes with a hood. You lose more heat underwater, mainly from your head (ca. 30%) so wear a hooded garment. The longer you stay under water, the more important proper dress becomes. We prefer an outfit like this:
These clothes are both warm and comfortable in the water,
but may slow you down a bit as the hoodie soaks up a lot of water, the tracksuit and anorak less so.
Wear all clothes first in the bathtub to make sure it all fits well when wet, before you wear it in the pool for training.