It is the Oliver Whitby Bluecoat School in Chichester, though we don't know why it's in our family (from West Wittering about 7 miles away), nobody recognises anyone in the picture.
It's a postcard style photo with a small rubber stamped label reading "The Westgate Studio, 40 West Street Chichester" on the back. I don' know the date, but would guess between 1910 and 1920.