Noriko was apparently well aware of Sensei and Maya's relationship back during the tutoring days:
Sensei didn't disappear until after his relationship with Maya already began.
Ami saw what's on his computer, phone, etc. She's not exactly hiding that she knows:
Niki is a maybe. She's joked about him flirting with kids at a playground:
As for Tsubasa, she's a maybe, but her 'The Lucky Few' Event, seems to imply she knows and is merely playing with him. Tsukasa may even be saying the whole husband thing because Tsubasa put her up to it. Perhaps a way to keep Sensei around and interested, since he's valuable to Tsubasa apparently.
Also, Maya seemed to convince Sensei to leave Noriko, almost 5 years ago. Why or How, isn't clear but it apparently came out of nowhere for Noriko, and seemed to be when he stopped tutoring, which Ami never understood:
