Translation:He is a student but they are not students.
It's an Irish thing, I'm not sure about the UK. My point was that at least both should be accepted. 'Pupil' is marked incorrect even though it would be a better translation into English as spoken in Ireland. And it might be helpful to note that you would not use the word 'dalta' in irish when referring to a university student, for example, you would use 'mac léinn'.
Since 64% of the people who are taking the Irish course are in the US, US English usage is relevant.