Theory: Those who don't 'love' Sensei, can see the cracks in time, or are willing to give it thought. (Yasu, maybe Touka, Tsuneyo, Yumi, etc). The rest are 'encouraged' to 'forget' or pass it off and Sensei being silly.
problem with that
this update Yumi figuratively said she loved sensei and had feeling for him up till he let nodoka do what she did to her
Yumi:
use your stupid fucking adult brain to put yourself into the shoes of a confused teenage girl taking the world on all by herself! imagine what it would be like finally being helped by someone after years and years of doing everything alone! imagine how you would feel about that person! then imagine what it would be like to have it all taken away! ifucking hate you but i dont okay, i dont!
or at least i didnt
(also consider her response when nodoka gave her the choice to sit and watch as she took advantage of the guy she 'doesn't' love or to masturbate . )
then there is
Otaha
who doesn't... hate sensei... but if there is a character that seems to not care it seems to be her at least in terms of apathy and at most jealousy
the girls that can see the 'cracks' are the ones most fractured and broken
yumi, tsuneyo, yasu could see them from the moment we met them in their own ways. nodoka does when she has a manic episode. miku is to incoherent when she has a panic attack to figure anything out about it and she represses it otherwise. Rin has show some surface level awareness when she cut.
ones who have started to see them because of sensei breaking them:
ayane, makato, sana
recall makoto full knows she tried to kill herself and it didnt work, she commented on it twice from what i recall
you could probably make several more cases for other girls too