I am looking for schools in the London area operating in the years 1870/71 at a minimum where Americans traveling at length abroad might have placed their teenage sons to further their education. Specifically schools where the boys could board. I also believe the school I am looking for would have been near or in the Brixton area or perhaps Brixton was part of the name of the school?
This list of English endowed schools by county from the early 19th century is organized by county/shire, including Surrey (location of Brixton) and the adjacent Buckinghamshire and Middlesex/London counties, which include among others the town of Eton and its eponymous College.
-- Wilson's Grammar School, Camberwwell (Most likely possibility - see below)
-- Farnham Grammar School
-- Royal Grammar School, Guildford
-- Kingston Grammar School
-- Saint Olave's Grammar School
-- Saint Mary Overey Grammar School
-- All Hallows Barking Grammar School
-- Charterhouse School
-- Christ's Hospital School
-- Mercers' Chapel Grammar School
-- Merchant Taylors' School
-- St. Paul's School, London
-- Westminster School
Wilson's Grammar School was located in Camberwell from founding until 1975. With the centres of Camberwell and Briston barely 2 km apart, that would likely be the school closest to Brixton at the time of interest.
Counties of South-East England:
Many thanks to the commenters below who assisted in correcting my faulty knowledge of South England geography.