junior has 4 distinct synonyms spread across 2 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. Its one recorded opposite is senior.
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.
younger; lower in rank; shorter in length of tenure or service
used of the third or next to final year in United States high school or college
“the junior class”
1 of the 7 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for junior.
Synonyms, antonyms and the broader and narrower relations for junior come from Princeton University’s WordNet. Full sources and licences.