Some of it is obvious, with Usagi and Mamoru getting into the exact same argument they did in Episode One. Others are Word of God:
"Why isn’t Usagi a third year in junior high school yet?
To which the following answer is given:
Normally, she would have definitely moved up a grade, but since she was once again reincarnated prior to April, she’s re-doing her second year of junior high. She didn’t flunk out, really!"
There are some theories that the year reset: https://www.tuxedounmasked.com/how-were-the-sailor-soldiers-reborn-after-the-fight-with-queen-beryl/
and there is evidence hinting at it.
Some of it is obvious, with Usagi and Mamoru getting into the exact same argument they did in Episode One. Others are Word of God:
"Why isn’t Usagi a third year in junior high school yet?
To which the following answer is given:
Normally, she would have definitely moved up a grade, but since she was once again reincarnated prior to April, she’s re-doing her second year of junior high. She didn’t flunk out, really!"