I mean, he was doing work with the student council while she was preparing for her class' maid cafe; I don't think that's all that unreasonable. Kanan obviously knew what he was doing since she went to the student council room, he just happened to be out of the room moving stuff at the time.
That's not it. Kanan and Kyougi are in the Disciplinary Committee. Hanako is in the head of the Culture Festival Committee (created just for the culture festival). Student Council is a separate entity from both of them.
Ever since chapter 1 they've been hanging out in the Disciplinary Committee room after school every day. The whole duel between Kanan and Tekka was to make him the member of her committee, so they could keep spending that time together.
Kanan went back to the Discpilinary Committee room as usual and for the first time ever he never showed up. She doesn't know he promised to help Hanako.
He left his girlfriend hanging and ditched his work in the Disciplinary Committee at the same time.