The schools do know the results in advance. They receive them by the close of the school day on Wednesday at the latest, but usually they arrive on Tuesday. The full process is described on pp 34 & 35 of this document. https://schoolsweb.buckscc.gov.uk/schoo ... R_2013.pdf
I think that also answers your question about the reason for the schools being told in advance?
The system for processing the results letters can vary, but usually the Head, or Deputy Head, plus the School Secretary will deal with the administration. Best practice, which I believe is observed in every school where possible, is that as few people as possible will see the results before they are released to parents.Please note that all of this only applies to Bucks resident children.
OoC children receive their results by post, directly from BCC. Until last year they were posted on the published results day, but last year they were posted to arrive
on results day.