Teachers rarely know the full story behind their students, and this is particularly so at Locke, in South Central, one of the city’s poorest and toughest areas. “So much goes on away from school,” says Ms. Levine, who loses students to homelessness, pregnancy, work, drugs and jail. She never knows which ones will make it through. Most don’t. The ninth grade at Locke four years ago had 979 students; in June, 322 graduated.