Hanai found out that Yakumo’s class is assigned to clean the pool, but he didn’t want Yakumo (he forgot that she’s not alone in it) to do such task. So he forced his homeroom teacher to assign pool cleaning to them. Tenma’s friends teased him about his hopeless crush on “Tsukamoto” Yakumo. Harima assumes this “Tsukamoto” is his crush and decides to _take care_ of Hanai. Thus starts the pool war.
I’m starting to think that Karasuma’s in the wrong series. T_T