Students at elite schools learn their accents from peers, not from the faculty or staff. It’s a well-known phenomenon, the premier example perhaps being the “Oxbridge” accent.
Children in general learn their language from their peers. That’s how it goes with immigrants, too: First generation is almost monolingual Old Country, second generation is bilingual Old Country and whatever the local dialect is, and third generation is monolingual local dialect. There are exceptions, but they are rare.