A “prep” school in the US or Europe is different from a Prep school in the UK, so I will simply touch upon boarding schools that offer education at the High School level, accepting that many of these also offer education to younger students as well. My reason for this being that the standard of academic excellence comes much more in to play at this level.
Britain and Switzerland tend to have the best collection of boarding schools in the world, the former being an extension of a longstanding tradition of academic excellence offered to well-to-do British families, and the latter catering for many years to the more international “jet set” clientele. Here are a few that come immediately to mind, as having maintained a standard of enviable excellence for more than a hundred years (some as much as 400 or more!):
The Twentieth Century has meanwhile seen the USA developing a roster of boarding schools that are proving competitive to these ancient European institutions: schools such as Phillips Exeter Academy, Lawrence Academy, Middlesex, Hotchkiss, and Miss Porter’s School, to name – again – but a few.
Ultimately, however, the mark of a great boarding school experience is a completely subjective one. Does your child thrive better in a single-sex environment or co-ed? Is a small campus better than a large one? Does the school offer A-levels, IB, or some other program? Is the housemaster/housemistress the type of person who would be a positive disciplinary, social, academic, and spiritual influence on your child? The list of criteria can seem endless. I would suggest, therefore (from first-hand experience), that you focus less on the arbitrary rankings of a website, magazine, or co-worker, and spend time identifying the “best” school for your unique and individual child. Whereas the pre-teen years are formative from a predominantly social perspective, the teen years are formative socially, intellectually, politically, and otherwise. What may therefore be a great school for child “A” may be quite inappropriate for child “B”. As long as the school is in the top 30, their ability to prepare a child for university entrance will likely be sound enough. Your choice thereafter will determine whether the 4-5 years your child spends in school are truly maximized for their own personal long-term betterment.