Turns out the one spreading the rumors was Hashimoto, who did so at the behest of Sakayanagi. Shortly thereafter, Sakayanagi appears before Class C and she informs them she has evidence regarding the rumors about Ichinose.
I know what crime it is you're guilty of.
أعرف الجريمة التي أذنبتِ بها
Sé qué crimen cometiste en el pasado.
Sé qué crimen cometiste en el pasado.
Eu sei de que crime você é culpada.