alma mater; one's old school
Refers to the school (elementary through university) one previously attended or graduated from. Carries a sense of personal connection and is used in both formal and casual settings.
He attended his alma mater's alumni reunion.
I met a teacher from my old school for the first time in a while.
After the graduation ceremony, I visited my old school for the first time in a while.
He now teaches English at his old school.
The graduates made a donation for the sake of their alma mater.
He returned to his alma mater in triumph with his gold medal.