This is a single arm, open-label, multi-center, phase II study to determine the efficacy and safety of tisagenlecleucel in de novo HR pediatric and young adult B-ALL patients who received first-line treatment and are EOC MRD positive. The study will have the following sequential phases: screening, pre-treatment, treatment & follow-up, and survival. After tisagenlecleucel infusion, patient will have assessments performed more frequently in the first month and then at Day 29, then every 3 months for the first year, every 6 months for the second year, then yearly until the end of the study. Efficacy and safety will be assessed at study visits and as clinically indicated throughout the study. The study is expected to end in approximately 8 years after first patient first treatment (FPFT). A post-study long term follow-up for lentiviral vector safety will continue under a separate protocol per health authority guidelines.