The institution is not a member of the Association of Business Schools and was previously known only to an insider circle of management consultants.
The Sainsbury's employees, ranging from directors to shopfloor workers, take six-month, part-time courses that are designed to change their behaviour toward one another in order to change their behaviour toward the supermarket chain's customers.
The unconventional training programme, run by the London-based Piper School, is part of a broader effort to overhaul the company's internal culture in the wake of a string of disappointing financial results.
Last month Sainsbury's announced plans to shake up its 3,000-strong head office, cutting 300 executives, many in the marketing department. The company's director of marketing, David McNair, and director of customer services, Mike Conolly, are both to leave as part of the changes.
"Sainsbury's is reorganising itself to sharpen the focus of its customer relations," said an industry insider. "To do that people must be trained."
In contrast to orthodox staff training, which takes place on company premises or at a fixed site outside, Piper School students roam retail venues. They meet at irregular times interspersed with their normal duties in groups of eight to 12. "Once you start going, you never really stop, because you never stop the need for learning," an insider explained.
Founded in 1997 by Crispin Tweddle, a management consultant and former director of the Fitch design consultancy, the school originally did the bulk of its business with Asda and Kingfisher. But over the past 18 months, Sainsbury's has been a prime customer.
"You can't get a place at the Piper School," said a retail industry analyst.
"All the slots have been taken by Sainsbury's."
A company spokeswoman declared: "We do spend a lot of time and effort on training." However, she dismissed suggestions that there was anything out of the ordinary about the company's patronage of the school.