As stated - Ruri's calculations are perfectly flawless, based on common sense.
Anyhow, she did not factor in uncommon nonsense.
I don't know how this will continue. I have my own ideas, but kisa likely planned something even more insane.
Well, let's hope Kanaria used her connections during Sou's absence and had the classroom upgraded. Like installing high performance air purifiers and such to counter the female scent tsunami. These girls might be able to fool Sou (he's either too dense to notice or flat out ignores it) but Sumire will sense it right away.
Also, I wonder if Sakura will be there. I can see Sumire tagging along since she goes to college. So she's free to join the fray but Sakura would usually have to go to school on a Weekday (it's Friday) - unless visiting the brother's school is a valid reason for her to be allowed to take the day off.