Here is the full list of all 9 Blooming Nightshade achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
You couldn't find or keep a romance alive until the end of the year...
Witnessed two of your friends become a couple.
Spend Christmas Eve with Shiori.
Spend Christmas Eve with Kiku.
Spend Christmas Eve with Atsuko.
Spend Christmas Eve with Nanami.
Spend Christmas Eve with Kiyomi.
Spend Christmas Eve with Izumi.
Got through the first month of the semester.