preparatory school
US /prɪˈpær.ə.tɔːr.i skuːl/
UK /prɪˈpær.ə.tɔːr.i skuːl/

1.
a private school for children aged 7 to 13, preparing them for public school
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He attended a prestigious preparatory school before going to Eton.
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Many children from wealthy families attend preparatory schools.
2.
a school that prepares students for higher education, especially for college or university
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She plans to attend a preparatory school to improve her chances of getting into an Ivy League university.
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The curriculum at this preparatory school is very rigorous.