WASHINGTON — Robert M. Bell was 16 years old when he recruited classmates to join a sit-in at a downtown Baltimore restaurant. The sit-in was Bell’s first, and he remembers being a little nervous. On the afternoon of June 17, 1960, the group entered Hooper’s restaurant and a hostess said she wouldn’t seat them. “I’m sorry, but we haven’t integrated as yet,” she said. The group pushed past her and sat down anyway. Police were called and 12 demonstrators, including Bell …