"High school students cannot smoke."
Can anyone please explain to me why there is a "は” in between "すって" and "いけません"?
高校生はすってはいけません should be a valid answer for this. Much like in English, when you say 'smoke' as a verb, it is automatically implied that you're talking about tobaco/cigarettes. Note that the English sentence does not say "High school students cannot smoke tobaco", because no one would bother including the 'tabaco/cigarette' bit.
I guess you could say it's just one of those things that differs between languages. In English, it's fine to just say "smoke" and people understand it as "smoke cigarettes". In Japanese however, you don't just say 吸う（すう）when talking about smoking cigarettes/tobacco, you always say タバコを吸う（すう）.