what im still scared about is: wtf happend to sumisumi? did the get a couple? did she manage to be happy like the rest?
To me it seems like she was the only one without a confirmed partner
I never fully got why the "desu" thing was seen as hyperformal and her verbal tick. Is it as simp[le as adding desu to "shoganai" or "shikata ga nai" etc? Maybe it's that she always puts a desu. Someone says kyo no tenki wa? and she always says "Atsui desu" instead of "atsui?" It's not as obvious as Eli's khorosho or Nico's nico or Rin's nyan or Kunikida Hanamaru's zuru. Or Marisa's za. Or whatever Momo's verbal tic was that ended up being rendered as "D00d!"
It's grammatical to add desu at the end if you wish, and it's not exactly keigo.
So I almost got it right What she actually does is use the perfectly ordinary but a bit teacher-y "no desu" all the time. So she would say "It's hot" as "Atsui no desu." which is translated in subtitles and scanlations as "It's indeed hot." So the others are teasing her for sounding teacher-y. And in general, while she doesn't quite use keigo, she does use old-fashioned words, and rarely uses slangy words. for instance, she never says "'n desu" always "no desu."
She didn't appear until Episode 11 and i didn't notice her language being weird or anything. High school girls are quick to pick up on small things, is my verdict. Also, SumiSumiKaichou gets no respect.