senior has 5 distinct synonyms spread across 3 of its 5 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. Its one recorded opposite is junior.
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.
older; higher in rank; longer in length of tenure or service
“senior officer”
used of the fourth and final year in United States high school or college
“the senior prom”
Synonyms, antonyms and the broader and narrower relations for senior come from Princeton University’s WordNet. Full sources and licences.
an undergraduate student during the year preceding graduation