。
She quit school for health reasons.
Literal
She [topic-は] health [in-terms-of-上の] reason [because-で] school [object-を] quit.
健康上の理由 is one of those bureaucratically pre-fab phrases that schools, hospitals, and HR departments lean on heavily — 'reasons of health.' The structure parses as 健康 + 上 ('with respect to') + の + 理由: 'reasons in the domain of health.' Other ~上の compounds work the same way: 法律上の問題 ('legal issues'), 仕事上の都合 ('work-related circumstances'). Causal で then marks 'because of (those reasons),' and 学校をやめる is the standard collocation for leaving a school. The whole sentence has the discreet, euphemistic feel of a written explanation rather than a casual account.