These are terms that are specific to cultures that have a strict sense of seniority, even among students in school. In the United States, no exact translations are available. Instead, you say things like: "This is a classmate from school" regardless of age. If you *have* to specify rank/seniority, you can say "This is an older (or senior) classmate" or "this is a younger classmate," but this is rarely ever specified unless the classmate's age is an essential part of the information.

Note that English does have the words 'upperclassman' and 'underclassman'. The former refers to students in their 3rd or 4th year, while the latter is for 1st and 2nd year students. The plural forms would end in 'men'. Generally, they include both males and females, but you will also see the forms 'upperclasswomen' and 'underclasswomen' used at schools that are all-female. Don't confuse these terms with 'upper class' and 'lower class' (note the space between words) which refer to social standing, i.e. rich and privileged vs. poor and disadvantaged.