i actually don't mind when translations use a name as the subject of sentences; that's what they're actually saying so it's fine (but slightly altering it to addressing them by name during the sentence is fine too). btw you can just say middle school third-year big dog, i think introducing US terms could just make it more complicated for international readers lmao
edit: oh i forgot to comment on the series lol. it's kind of funny and silly that they seem to be close friends already, confiding in each other-- but both haven't realized it and think they're just acquaintances. that's what really caught my attention, so i hope the author meant for that to be the premise of the series and it's not just incidental to the first chapter!