I recently saw this scene in the visual novel, although it's been a while since I watched the anime. But what happened in the visual novel was Chisato texted Oojima asking where he was, to which he responded with a lie saying he was out with his mother, when in fact he was with his teacher. He did this in order to stop Chisato from pestering him about his whereabouts because she's possessive like that. The teacher isn't his mother, but the hugging and the text were probably why you're thinking that.