It’s Friday afternoon. Students in various shades of preppy colors are pouring out of the university’s gates for the weekend. The patios in front of the restaurants are buzzing with cocktail chatter; there’s a long line at the movie theater. But I am climbing a rickety staircase in a two-story clapboard building just off Nassau Street. There, amid the disarray in a rabbit warren of small offices, I will talk to two university students who once a week leave their manicured campus to spend a day behind bars tutoring young men and women who are their age but who are held in a nearby prison.