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Not all students at our school go on to university.
Literal
This-school [attributive-の] students [attributive-の] all [subject-が] university [target-に] advance [the-case-that-わけ] [is-not-ではない].
Built on ~わけではない, a nuanced partial-negation construction meaning 'it's not that / not exactly / not necessarily.' Here: 'it isn't the case that all students advance to university' — i.e. some do, but not all. Contrasts with flat negation 全員が進学しない ('none advance'). 本校 is a formal, stiff word for 'our school' used in official announcements and institutional contexts. 進学する is a set verb meaning 'advance to higher education.'