"High school students cannot smoke."
Duolingo insists 高校生はタバコをすえません is correct, but there's no すえ option on the cards thing (at least for me). If I skip it shows 高校生はタバコをすってはいません, however there's no option for はい either.
Possibly because you left out the け in いけません? The cards I was given just now included て、は、いけ、ませ and 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.
?すう（吸う）means to suck. タバコを吸う means to smoke. If you say 高校生はすってはいけません it just means "High school students cannot suck."
According to my dictionary that is not true
吸う=1 einatmen; aspirieren. || einsaugen; lutschen. 2 rauchen. 3 aufwischen; aufsaugen lassen. 4 küssen. 5 anziehen; an sich heranziehen.
吸う=1. to smoke; to breathe in; to inhale 2. to suck; to sip; to slurp 3. to absorb; to soak up 4. to kiss
吸う means "to smoke" when used with an object that is being smoked (burned and inhaled, it does not refer to preserving meat). It cannot be used intransitively. It is a transitive verb that requires an object to be grammatically correct.
Can anyone please explain to me why there is a "は” in between "すって" and "いけません"?
Ditto here. There is already a は in the sentence at the start. I haven't encountered this kind of dual subject sentence before
the whole ending はいけません added to theて form is the ending for a negative command. I am assuming that is is pronounced ha not wa because it is not a particle.( total beginner disclaimer)
The English sentence doesn't include specifically smoke tobacco, so I didn't include it. I know "すう" can also just mean "inhale, breathe," so does "タバコ" need to be included for that reason?
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）.
吸う is a transitive verb. It requires an object to be grammatically correct. It cannot be used alone, unlike the English verb "to smoke".
Japanese nouns do not always have to specify the plural, the plural can be implied in most cases.
High school students are physically unable to smoke cigarettes? They do in the United States, but not legally.