Telehealth for Children’s Access to Services and Treatment Act or the TeleCAST Act This bill requires the Government Accountability Office to report on children's access to substance use disorder services under Medicaid. The report must include an analysis of: (1) state options to improve access, such as via telehealth services at school-based health centers in underserved areas; and (2) reimbursement rates for substance use disorder services. In addition, the Centers for Medicare & Medicaid Services must report on ways to reduce barriers to the use of telehealth services for children with substance use disorders under Medicaid. The report must include an analysis of the differences between telehealth and in-person services with respect to specified metrics (e.g., costs and quality of care).