An apple a day (keeps your crush not far away)

Harry's class starts learning about ancient Greek culture in muggle studies, and discovers the tradition of throwing an apple to confess your love, and if the person caught it they were accepting their love.

Riddle does, too, and proceeds to start aggressively throwing apples at Harry during breakfast, lunch and dinner, and Harry keeps dodging them in confusion thanks to his seeker reflexes, and then starts throwing apples at Riddle thinking it was a food war, oblivious to the implications of it all.