Meisha Porter, a longtime Bronx educator whom Mayor Bill de Blasio named on Friday as his choice to replace Richard A. Carranza as schools chancellor, began her path to the chancellorship as a teenage activist who caught the attention of a group of urban planners in the South Bronx in the early 1990s.

Ms. Porter, who will be the first Black woman to lead the nation’s largest school system, was a youth organizer in the Highbridge neighborhood, and Richard Kahan, who was coordinating the planning for a 300-block area of the community, invited her to a meeting with local leaders at the Bronx borough president’s office.