principal has 12 distinct synonyms spread across 5 of its 7 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.
the original amount of a debt on which interest is calculated
the educator who has executive authority for a school
“she sent unruly pupils to see the principal”
capital as contrasted with the income derived from it
the major party to a financial transaction at a stock exchange; buys and sells for his own account
Synonyms, antonyms and the broader and narrower relations for principal come from Princeton University’s WordNet. Full sources and licences.