The reasons for the strong Swedish dominance are as explained by Richard Sparks manifold; suffice to say here that there is a long-standing tradition, an unsusually large proportion of the populations (5% is often cited) regularly sing in choirs, the Swedish choral director Eric Ericson had an enormous impact on a cappella choral development not only in Sweden but around the world, and finally there are a large number of very popular primary and secondary schools (music schools) with high admission standards based on auditions that combine a rigid academic regimen with high level choral singing on every school day, a system that started with Adolf Fredrik's Music School in Stockholm in 1939 but has spread over the country.
Which music school began the implementation of high musical standards? (If the question is unanswerable, say "unanswerable")
Adolf Fredrik's Music School