Why can't this be "en elev" ?
It can be. Afaik, in Commonwealth English, "en elev" is a pupil and "en studerende" is a student, but in American English, we don't differentiate the two.
Reported: "en elev giver hende et rødt æble" should be accepted.
Also reported, better chance of an elementary school student bringing an apple that someone at a university.
The hint says that its acceptable to not add "En" at the beginning, that "studerende" alone translates to "a student" which makes no sense. Not looking for an explanation, just letting everyone know the hints are wrong.