This is the 4402nd most frequent Korean word.
학교에서
"At school" or "in school."
Here, 학교에서 describes the origin of the events being discussed, meaning 'at school'.
학교에서 일어난 일을 친구들과 얘기했다.
We talked about what happened at school with my friends.
In this sentence, 학교에서 refers to the eventual location where the prepared items will be used, meaning 'at school'.
학교에서 사용할 물건을 준비했다.
I prepared items to use at school.
학교에서 indicates the location where the studying activity happens, meaning 'at school'.