Even most investment banking success stories still built a good foundation in finance through the best business school or degree in finance, economics, or business. For example, Sarah Mitchell, Managing Director at the largest firm, started with the Harvard Business School degree. According to her, getting the technical skill for dealing with complicated financial models and presentations to clients was through education and early internships and thus prepared her for investment banking.
The hands-on experience was gained from the internships. David Chen is a senior partner in an investment bank globally, but he can recall how the internship in a boutique firm made him learn the details of deal-making and how to treat clients. It made him stand out when the firms were hiring him full-time and developed in him the work ethic to make it through a long career in investment banking.