I wish that this would be updated to be more consistent, as well as grammatically correct in English. Sometimes the plural command is marked wrong if you don't include "You all", whereas other times including "You all" is marked wrong. First of all, it shouldn't be required, because it is completely unnatural for many, if not all, English speakers. Second, if it is included, it needs to have a comma after it in order to be grammatically correct. Without the comma, it's not a command in English.
"Walk slowly" has to be accepted here, because there is a very high probability that is exactly what would be said in English, whether addressing a single person or a group. If Duo wants the plural it will have to indicate that by supplying context. Mwalimu aliwaambia wanafunzi: "Tembeeni polepole!"