While sensei is clearly hiding something, its doesnt seem like shes Rum.
Even though Haibara is aware of her eye situation, she doesnt feel suspicious of her at all.
Usually someone as cautious as Haibara would instantly think shes suspicious, and she can sense black organization members as well.
Sensei just must have been involved in a serious case, one that the superintendent is aware of, who by the way I also do not think is Rum.