Pembrolizumab in Treating Younger Patients With Recurrent, Progressive, or Refractory High-Grade Gliomas, Diffuse Intrinsic Pontine Gliomas, Hypermutated Brain Tumors, Ependymoma or Medulloblastoma

Inclusion Criteria: * INCLUSION CRITERIA FOR STRATA A, B, D AND E * Tumor: patient must have one of the following diagnoses to be eligible: * Stratum A, currently closed to enrollment: Patients must have a recurrent, progressive or refractory DIPG following radiation therapy with or without chemotherapy * Histologic diagnosis is not required for patients with typical imaging findings of DIPG (defined as patients with a diffuse expansile mass centered in and involving at least 2/3 of the pons); patients with brainstem tumors who have undergone biopsy with a diagnosis of high-grade glioma or diffuse infiltrating glioma are also eligible * Stratum B: Patients must have a histologically confirmed diagnosis of a non-brainstem high-grade glioma (NB-HGG) that is recurrent, progressive or refractory following therapy which included radiotherapy; spinal primary disease is eligible * Stratum D: Patients must have a histologically confirmed diagnosis of ependymoma that is recurrent, progressive or refractory following therapy which included radiotherapy * Stratum E: Patients must have a histologically confirmed diagnosis of medulloblastoma that is recurrent, progressive or refractory following therapy which included radiotherapy * Patients must have adequate pre-trial formalin-fixed paraffin-embedded (FFPE) tumor material available for use in the biology studies mutational analysis and genome wide sequencing for each stratum * Patients with DIPG who have tissue available are requ…