And (X) As (Y)

Takaaki Fujimura is an average Japanese high school boy, going to an all-boys' school. Everything is the same routine, day after day, until he finds Valentine's chocolates in his shoe locker two days after Valentine's Day with a note that simply says "AND(X)AS(Y)". He continues through his normal routine, but wonders... who sent him that chocolate? Is it the girl from the neighboring school he rides the subway with, or perhaps even one of his own classmates from his all-boys' school?