I don't know if it is Lily. She's shown to be pretty intelligent and not someone who would necessarily need to cheat in order to get good grades. But I don't think we actually know how well she's doing in school, objectively. Anyway, the fact that Derek is on the list of the students that Bella considers to be in her top 3 should tell you that it could actually be Lily, since Bella wouldn't have a clue that she's cheating even if she is. (Or didn't have a clue, before her and Professor Hoff started looking into it.)