Loooool
So many possibilities, was she one of his bullies? Was she just there watching him getting bullied? Was she the bullied? Did he step in when she was being bullied and that lead to the bullies to go against him? One thing is sure, her love to him came from that time and her face cracking again in despair is near to appear, oh god i love when that happens in all the mangas, when they lose their composture thats why i love this kind of mangas