While considered "alternative" today, most alternative systems have existed since ancient times. After the public school system was widely developed beginning in the 19th century, some parents found reasons to be discontented with the new system. Alternative education developed in part as a reaction to perceived limitations and failings of traditional education. A broad range of educational approaches emerged, including alternative schools, self learning, homeschooling and unschooling. Example alternative schools include Montessori schools, Waldorf schools (or Steiner schools), Friends schools, Sands School, Summerhill School, The Peepal Grove School, Sudbury Valley School, Krishnamurti schools, and open classroom schools. Charter schools are another example of alternative education, which have in the recent years grown in numbers in the US and gained greater importance in its public education system.

Answer this question, if possible (if impossible, reply "unanswerable"): What aren't some examples of alternative schools?
unanswerable