 ARM II: Patients receive vincristine sulfate IV on days 1 and 57; dexamethasone PO on days 1-5 and 57-61; mercaptopurine PO QD on days 1-84; MTX PO on days 8, 15, 22, 29, 36, 43, 50, 57, 64, 71, and 78; methotrexate IT on day 1; and nelarabine IV over 60 minutes on days 29-33. Treatment that includes nelarabine repeats every 84 days for 3 courses. Patients then receive treatment without nelarabine as follows: vincristine sulfate IV on days 1 and 57; dexamethasone PO on days 1-5, 29-33, and 57-61; mercaptopurine PO on days 1-84; methotrexate PO on days 8, 15, 22, 29, 36, 43, 50, 57, 64, 71, and 78; and methotrexate IT on day 1. Treatment without nelarabine repeats every 84 days until the total duration of study treatment is 2 years from the start of interim maintenance therapy approximately week 121 for girls with T-ALL and 3 years from the start of interim maintenance therapy approximately week 173 for boys with T-ALL. 