An upperclasswoman is a female student in one of the last two years of an American high school or university — a junior or a senior. It is simply the female form of 'upperclassman', built the same way and used the same way, and its opposite is 'underclasswoman'. The plural is 'upperclasswomen'. You will meet it far less often than 'upperclassman', for a plain reason: when people talk about a mixed group, English has long used 'upperclassmen' for everyone, so 'upperclasswoman' tends to appear only when the speaker is pointing specifically at a woman — in a girls' school, in women's sports writing, or where the contrast with the men matters. Many American schools now avoid both and say 'upperclass students' or just 'juniors and seniors', which covers everybody and is shorter. So this is a word to recognise when you read rather than one you need to reach for. As with 'upperclassman', the 'class' here means the year group you belong to, not money and not a classroom, and it is written as one word.

（美國高中或大學的）高年級女學生