Brenda Ellis's day begins at 6:30 a.m., when she rousts her 11-year-old son, Imani, from bed, hustles him into the kitchen for breakfast and to the school bus by 7. Tianna, 13, and Dikia, 17, quickly follow. Then she's off, some days to a substitute-teaching job in Prince George's County, others to tax clinics for the working poor, where she is earning credit for a hoped-for career in accounting. If it's a school night for her, Ellis, 41, rushes home to her Landover apartment to quickly fix dinner before bolting out at 5:45 p.m. for the half-hour drive to Strayer...