Cloth masks protect other people, not the wearer. And teenagers are vanishingly unlikely to suffer any ill effects from the virus if they catch it. Badly used cloth masks can make the wearer more likely to get the virus.
Masks are uncomfortable, irritating, can cause acne, dry mouth, bad breath and other minor irritations. They interfere with social interaction and learning. But not seriously in most cases.
Which means there is a philosophical question. How much discomfort and annoyance is it reasonable to cause someone (require someone who is legally obliged to be there) to protect someone else? Teenagers have already suffered socially, financially (they will struggle to recover in their future work lives from this), educationally... All to protect sections of society that refer to them as snowflakes and have bagged all the good housing. They have already suffered for older people.
The question is, how much is it reasonable to require of them going forward? To protect others. It's easy to require altruism in others. But Brexit showed that when it's the other way round, screw the young.