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.
If you lose, I will reveal to everyone 
that you're the mastermind behind Class C.
إن خسرت، سأكشف للجميع أنّك 
العقل المدبّر للفصل ج
Se você perder, revelarei a todos que
você é o cérebro por trás da Turma C.
я всем расскажу, что именно ты
руководишь классом из тени.
คุณต้องเปิดเผย
ว่าตัวเองคือคนที่อยู่เบื้องหลังห้อง C