While the K-12 space has some flexibility with regard to teacher credentialing requirements, charter and private schools have even more latitude. This is partly because these schools are set up differently.
Like public schools, charter schools are publicly funded, tuition-free schools, although they don’t always receive the same amount of funding as public schools. Furthermore, they operate according to individual agreements (aka “charters”) with state or local governments, which outline things like operational rules and performance standards. As a result, some charter schools don’t require all teachers to hold a state license or certification.
Private schools, meanwhile, can choose not to receive any government funding, which frees them from restrictions pertaining to curriculum, admissions and, yes, teacher certification. Teaching requirements are up to the state in which the teacher is practicing, but private schools may elect not to apply to be state chartered, meaning they’re not required to have teachers with licenses teach in their schools.
Of course, that freedom can work the other way too. Some private schools require that teachers not only be certified but also hold a postgraduate degree, such as a master’s degree or doctorate.
Charter and private schools, depending on their focus, may prefer their teachers to be subject matter experts rather than in possession of a general teaching certificate. The highly performing BASIS Charter School system is one example of this approach. Its teachers hold bachelor’s degrees or higher, but teacher certification isn’t required.