It probably means someone who attends school in the evenings after work (i.e., "night school").

Adult students who attend school/college/university after working during the day say they are going to "night school". My guess is that these students would call themselves and be called "night students". "Night school" can also be during the weekends, it just means that you are attending school in addition to your day job. I hope this answer helps.

Most students typically take classes during the day. A night student is someone who takes classes (of any kind) at night.

I mean i can say something about that. If a student going to university and get night education, it is called "evening education ". Also, there is a phrase as "night owl". This phrase is usually used for those who study, work, read at night.