When my brother was in seventh grade, his bio teacher got a guy who suffered from lung cancer and now has a stoma to come speak to them. A lot of good that did. My brother smokes.
You’re right, smoking is a dangerous habit. The yeshivas have so many things that they’re fighting against, they need to chooses their battles. Unfortunately, the spiritual risks of the world today, are very great and yeshivas have their hands full just keeping their boys “here.” If they smoke when they’re here, well, at least they’re here and not there.
~a~