Rachel Mejia has to raise her voice to be heard over the din of children running into St. Stephen’s cafeteria in the South End. The 17-year-old works with local kids a few days each week after school — helping with homework, being a buddy during playtime, and acting as a sounding board if someone needs a friendly ear. Mejia’s proud of her job, funded by the city as part of an afterschool jobs program for teens and young adults.