Virgin Snow ~Yuki Furu Oka de Anata to~

There is a legend in this private school that couples giving promises at an old chapel are watched over by the snow spirits and live happily forever.
This story begins when main character receives an anonymous conservative love letter not far from graduation. Is it just a prank or real love confession. He has three weeks before St. Valentine's Day to find out.

[From vndbreview]