A Safety and Efficacy Study of Combined Fianlimab + Cemiplimab in Children and Young Adults With Recurrent or Progressive High-Grade Glioma or Posterior Fossa-A Ependymoma

Key Inclusion Criteria: 1. Participant must be diagnosed with recurrent/progressive HGG or PF-A ependymoma with unequivocal progression on Magnetic Resonance Imaging (MRI) as described in the protocol 2. Participant must have histologically confirmed (at initial diagnosis or relapse) HGG or PF-A ependymoma 3. Participant must be an adequate medical candidate for surgical resection as described in the protocol 4. Karnofsky Performance Status (KPS) score ≥50 (in participants ≥16 years) or Lansky Performance Status (LPS) score ≥50 (in participants \<16 years) as described in the protocol 5. Adequate organ function as described in the protocol Key Exclusion Criteria: 1. Active autoimmune disease requiring systemic immunosuppressive therapy in the past 2 years 2. Active, serious medical illness, infection or other systemic illness which would limit participation in the trial 3. Has not yet recovered from any acute toxicities resulting from prior therapy 4. History of myocarditis 5. Prior treatment with antibodies to Programmed Cell Death Protein -1 (PD-1), Programmed Cell Death Protein Ligand -1 (PD-L1), Lymphocyte Activation Gene 3 (LAG3), or Cytotoxic T-Lymphocyte Associated protein 4 (CTLA-4) 6. Treatment with high dose systemic corticosteroids as described in the protocol 7. History of interstitial lung disease (eg, idiopathic pulmonary fibrosis, organizing pneumonia) or active, noninfectious pneumonitis that required immune-suppressive doses of glucocorticoids to assist with management Note: Other protocol defined Inclusion/ Exclusion Criteria apply