IN a one-room rural schoolhouse an hour’s drive from this city in a coffee-growing region of Colombia, 30 youngsters ages 5 to 13 are engrossed in study. In most schools, students sit in rows facing the teacher, who does most of the talking. But these students are grouped at tables, each corresponding to a grade level. The hum of conversation fills the room. After tackling an assignment on their own, the students review one another’s work. If a child is struggling, the others pitch in to help.