If neither bank nor school is on a corner, The bank is across the street diagonally from my school. If the bank and the school are at an intersection diagonally from each other at the intersection, you can say The bank is [catercorner(ed) / cattycorner(ed) / kittycorner(ed)] from my school. Of these choices I hear "kittycorner" most, but that may be a regional term of American English.
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