For decades, it has published definitive guides to fitting in with high society.
Now Debrett's has launched an etiquette class for the 21st century - instructing teenagers on the dos and don'ts of social media.
Its first lesson is to be wary of oversharing your lives on Facebook and Twitter.
The classes, costing up to £750 (NZ$1800) per person, are aimed at 13 to 16-year-olds and will point out the pitfalls of "living lives online."
The advice forms one module of a course called Coming of Age, which also covers how to interact with adults and what to pack when leaving home.
Debrett's, which publishes a range of etiquette books, said: "We share so much information and network through many social media channels - often to the detriment of face-to-face skills."
The course will ask youngsters to examine their social media profiles and see what information is available to the public.
Tips include not pestering friends to join groups or forums and replying promptly to comments or tweets. It also advises that cards or phonecalls to friends on their birthdays mean more than an online post.
James Field, training manager at Debrett's, said: "When we put our lives online, we are creating an idea of the sort of person we might be for others out there."
- Daily Mail