The south is rather notorious about this, especially in the form of being "right to work" states, where bosses can fire workers for pretty much any reason.
New York has a few companies that do it, and so does every Otis Spunkmeyer factory.
I just think if they are going to tell you that you can't smoke at home on your own time, then it just shouldn't be allowed period.
The school is obvious with why they are doing this, Smoking is bad for you. If you smoke you are a health risk, and when you become sickly the school has to pay for your out days, and the subs, and the everything else.
Just to keep this on the smoking topic lets look at other things that this can lead too. Say you made a mistake as a kid, you smoked, but you quite, because you didn't want to hurt your body that way anymore. Well now you have quite smoking, but you have a history as a smoker. You have the potential to start smoking again, and possible health risks that are associated with having smoked at one point in time.
They can claim the same thing, "They are a health risk because they have been smokers in the past," once again they can deny you the job, and despite that you stopped smoking you wont have a chance for the employment.
What about the kids that have parents that smoke, second hand smoke is just as bad, if not worse, and even though they had little to no control over it, and they now have a history of regular exposure to smoke, and also a health risk. Can they deny them as well?
Smoking is bad, that is the bottom line, but if they are allowed to claim you are a health risk and keep you from working, then once the whole first hand current smokers are no longer a problem, what about the additional possibility of them moving on to other things besides smoking, like previous cancer patients, age factors, diabetes, and obesity among other things.
Someone can also claim discrimination, being a smoker does not inhibit your ability to teach, it can mess up your health, and cause the school to be looking for a different teacher sooner, but doesn't change that you are a still licensed teacher, and qualified for doing the job.
If you don't want them to smoke, and enough companies are doing this, and if the government wants to allow them too, then they should make it illegal. Not try to be sneaky, because being sneaky can only mess with more people, and if you start with a ban on smoking what is to stop it from snowballing to include the health risks previously mentioned.
If the health risk is costing so much money that you wont give these people jobs, then just make it illegal. It isn't helping anyone, besides the people that make tobacco products.