' At ' is a point reference, so it is used when the place is so far away that it is conceived as a point or when its dimensionality is otherwise unimportant: I was at school all day. My plane landed at Paris en route to Berlin. ' In ' is a 2- or 3-dimensional place reference, so use it when you are conceiving of the location's dimensionality or are constrasting its inside with its outside: I was in school until I graduated in 1960.
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