As someone who IS a teacher at a school in Tanzania, I disagree that "the students are doing cleanliness" shouldn't be accepted. This is how it is said in Tanzanian English. Everyone here uses the phrase. In the context of a boarding school, it's usually referring to a period of time in which all the students are required to be cleaning their assigned parts of the campus.
The other translations ("the students are cleaning", etc.) should be accepted as well, but "the students are doing cleanliness" is exactly how it's translated into English here in Tanzania.
For me, when learning a language, it's much more helpful to translate literally, even if it sounds awkward in English, because it helps me to get the rhythm of the new language by seeing the word order and expressions in English. I wish duolingo allowed this more.