학교에 갔어요

We usually put the word '에서' back of the place such as 학교, 도서관, 회사 등등. We don't care how big the place it is, so 에서 can be the preposition like at, in. But 에 is differently used. It is attached to the word of place, but it can be ~toward or to the place. 오늘 학교에/학교를/학교로 갔어요. These are all correct sentences. If you say, 오늘 학교에서 갔어요, then native Korean may wonder where you went from school?