@dagmaraka,
these students are adults who smoke in their free time outside school premises -- clearly NONE of my official business. informal way is the only way in this case. if i did lecture, they would laugh it off and it could potentially become even 'cooler' than it is now. won't go there.
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You had stated that the problem is peer pressure turning non-smokers into smokers within a very short time frame.
Now no matter what the legal position might be they are clearly not adults in the developmental aspects of life as adults do not allow, in my opinion, peer pressure to run their lives. That behavior is for teenagers that had not reach adulthood yet.
If they are willing to bend to peer pressure, I see no reason why they would be resistant to adults in positions of respect telling them the facts of how stupid they are being or would be if they gave into peer pressure.