Translation:High school students cannot smoke.
It used to be very common in Germany. In the middle school grades (up to 10th grade) it wasn't accepted due to child protection laws, but in high school (11th to 13th grade) it wasn't a big deal and students sometimes smoked with teachers.
Fortunately a lot fewer people smoke nowadays.
te-form + "wa ikemasen" is a construction that means you cannot/ may not do something. For example --> from TABERU - tabete wa ikemasen (you may not eat). from KAIMASU - katte wa ikemasen (you may not buy). In this sentence, we are using the verb "SUIMASU", so it becomes sutte wa ikemasen (they may not smoke).
Correct answer they showed me: "High school students are not allowed to smoke cigarettes". I omitted "cigarettes" So in the answer above "smoke" is ok, but in the version I attempted you have to specify cigarettes. I know this is automatic marking and free, so you can't expect everything, but it is still frustrating