Chloe's cousin Victoria McEvoy (Ravenclaw 5) found out from Ella Vaisley (Slytherin 5) that she and Evan had sex when they were 12/13, and will turn around and tell Chloe this week. Chloe's going to tell G7 (because she does that), and at that point it can be assumed that it's become a rather widespread rumor and your character would know. If your character would like to be responsible for having spread the rumor, let me know! Otherwise we can blame it on Victoria.