school day has 2 distinct synonyms spread across 1 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
any day on which school is in session
“go to bed early because tomorrow is a school day”
the period of instruction in a school; the time period when school is in session
“stay after school”
Synonyms, antonyms and the broader and narrower relations for school day come from Princeton University’s WordNet. Full sources and licences.