Their parents event web page - https://www.kedlestongroup.com/our-locations/schools/olsen-house-school/news-and-events/news/olsen-house-school-may-2025-parents-event - lists their address as 85-87 Liverpool Road, Great Crosby.  That tallies with where it's currently mapped.

They'll be using the "Sefton" designation because that can refer to the whole council area (covering Bootle and Southport too) rather than just the village itself.