I stayed home from school because my mother was sick.
Literal
Mother [subject-が] sick was [because-ので], I [topic-は] school [obj-を] rested.
学校を休む is literally 'rest school' but idiomatically 'miss school' or 'stay home from school.' 休む is a verb that takes unusual object marking — you 'rest' the thing you're absent from rather than being absent 'from' it: 会社を休む ('take a day off work'), 授業を休む ('skip class').