Please do not post if you are not sure of the answer to a question. Distance from home to school is emphatically NOT measured on an "as the crow flies" basis. Although RGS is a Foundation School, and therefore sets its own admission rules, it largely follows BCC's own rules. BCC's guidance (linked within the page guest55 linked to!) explains, for RGS:
Where oversubscription occurs … then places will be allocated according to the distance between the family’s normal home address and the school’s nearest entrance gate: closest first. We will use the same distance definition and measuring system as used by Buckinghamshire County Council.
and the measuring system used by BCC is the "shortest appropriate route":
Shortest appropriate route - the shortest appropriate route between a preferred school and your normal home address will be measured by computer, using our geographical information system. We use a database that combines the Ordnance Survey ‘Integrated Transport Network’ (ITN) data as at 17 November 2006 plus a database of footpaths, entered on our system before 17 November 2006, which have a proper made-up surface such as tarmac or concrete. If you think that your child’s ‘shortest appropriate route’ may include footpaths or roads which may not already be included in our database, please check this with us before 17 November 2006.
To the original poster - you might find that asking the school or BCC directly would have avoided this misinformation.