"The students are working."
"Studentki" is a group of female students. "Studenci" is a group of male students, or just a mixed group.
"ci" isn't a translation of 'the' ('the' doesn't translate), it's just an interpretation, it's accepted. Because often in fact when you say 'the' in English, the meaning is closer to "these". But technically it's "these students". So if you omitted it, you were actually closer to the original sentence.
"ci" is the masculine personal form of "these". "te" is the not masculine-personal one. So if you went with "studentki", who are a group of women only, the right determiner (if you decide to use a determiner at all) is "te".
It's possible that Duolingo suggested another answer to you: "uczniowie". "uczniowie" (masculine personal) and "uczennice" (not masculine-personal) are 'students' of primary/middle/high school, before university. "studenci" and "studentki" are university students. American English doesn't have this distinction, British English would use "pupils" for the ones before uni.
Ergo: if you treat them as university students, the best answers are "Studenci pracują" and "Studentki pracują", the acceptable options are "Ci studenci pracują" and "Te studentki pracują" (These students are working).