〜では: as for in / at (place, group)

で plus は: the place or setting where something holds becomes the topic: "in this place", "at school", "in my country".

学校では先生と話します means "at school, I talk with the teacher". It marks where an action happens (で), then sets it apart as the topic. Do not confuse it with ではありません, the long form of じゃありません (see 〜じゃありません), or with sentence-initial では, "well then".