fellow student; schoolmate; alumnus of the same school
Refers to someone who studied under the same teacher, attended the same school, or belongs to the same academic lineage. Often used in formal or nostalgic contexts.
He is a fellow alumnus of my university.
A senior from the same school introduced me to a job.
Compound of 同 (same) and 門 (gate, school of thought), literally 'same gate', referring to those who entered through the same gate of learning.