student taking entrance exams while still in school; student who passed entrance exams on the first try
Refers to a student who takes or passes university entrance exams while still enrolled in high school, as opposed to a 浪人生 (rōnin) who has already graduated. The second gloss is a common extended meaning.
See also: 現役
He passed the University of Tokyo entrance exam as a current high school student.
I want to get into my desired university while I'm still a high school student.
Compound of 現役 (gen'eki, 'active duty' or 'currently enrolled') and 生 (sei, 'student').