all students in a school; whole student body
Refers to every student enrolled at a particular school, often used in announcements, events, or statistics.
All the students in the school gathered in the gymnasium.
The news spread to the entire student body.
Also means 'all students,' but 全校生徒 explicitly includes the school context (全校 = whole school). 全生徒 can be used more broadly without specifying a single school.
Compound of 全校 (ぜんこう, 'whole school') and 生徒 (せいと, 'student').