Logically, for all that happened so far would be really suitable for a successful confession next. Unless Serizawa has some trauma or complex, wouldnt really make sense to reject her after all his reactions and actions. But we all know mangas often can be surprisingly ilogical.
On a side note, theres a big chance that the boy from middle school might have feelings for her. Tho considering her desires for their "rules" she never fall in love before. So i suppose it wont be a problem.
@Pachka84 if u that bothered by this u're more than welcome to read something else. Such silly reasons tbh.