You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Private_school

Context:
In the United Kingdom and several other Commonwealth countries including Australia and Canada, the use of the term is generally restricted to primary and secondary educational levels; it is almost never used of universities and other tertiary institutions. Private education in North America covers the whole gamut of educational activity, ranging from pre-school to tertiary level institutions. Annual tuition fees at K-12 schools range from nothing at so called 'tuition-free' schools to more than $45,000 at several New England preparatory schools.

Question:
What are private schools that charge no tuition called?

'tuition-free