 ARM III: Patients receive high-dose methotrexate HDMTX IV over 24 hours and vincristine sulfate IV on days 1, 15, 29, and 43; mercaptopurine PO on days 1-56; and methotrexate IT on days 1 and 29. Beginning 42 hours after the start of HDMTX, patients also receive leucovorin calcium IV or orally once every 6 hours for 3 doses. 