The overall goal of this study are to: Part 1 - Find the best dose of ruxolitinib to use with standard chemotherapy in children, adolescents, and young adults with HR B-ALL; Part 2 - Find out if the investigational drug ruxolitinib helps chemotherapy to work better and to compare the effects, good and/or bad, of using ruxolitinib with chemotherapy. Other goals of this study are to better understand the effects of ruxolitinib in children, adolescents, and young adults who have HR B-ALL.