This is the 514th most frequent Vietnamese word.
cựu
"Former" or "ex-"
In this sentence, "cựu" is used as an adjective meaning 'former,' describing someone who used to hold a specific role or title, here as a soldier.
Anh ấy là một cựu chiến binh từng tham gia chiến tranh.
He is a former soldier who participated in the war.
Here, "cựu" again means 'former,' indicating a past association with a particular role, used for students in this case.
Bà ấy tổ chức buổi gặp mặt các cựu học sinh của mình.
She organized a meeting for her former students.
In this context, "cựu" still conveys the idea of 'former,' referring to individuals with a previous position of leadership.