"Middle school students"
When I got this wrong, it told me I was supposed to put "中学生達" but Duolingo did not give me "達" as an option and has also not taught me that symbol.
達 = たち (tachi). it's the plural marker but i've never seen the kanji for it used in this course as of yet so i'm not sure why it told you to use that...
I understand it's used here to teach the usage of 達, but it's not even necessary, unless you specifically want to emphasize that it's plural.
When your answer isn't counted correct, it picks one from the database to show you, and the choice can be pretty random. 中学生達 is an accepted translation because it's correct, and those who happen to have the knowledge may want to enter it. That also just happens to mean that it can show up when a user's answer happens to have been rejected.