people who are "upperclassmen" aren't immature 9th graders. they are seniors and juniors, or the top grades in a school. everyone other than the youngest. freshman in high school are not upperclassmen, and shouldn't call themselves upperclassmen. 6th graders are basically the same social status as 9th graders, if not better.
"wow 7th grader you can't be mean to me. you're talking to an upperclassmen"-freshman
"actually you arent cool and your a freshman, not an upperlcassmen. you aren't a senior or junior, yeah your not coool."-7th grader and the rest of the world