She couldn't have been tutored since six, since Sensei left Niki ~eight years ago, when Noriko was 8-9. Which, well, doesn't leave a lot of time for Noriko to be accepted back into Sensei's life, spend some time being tutored mostly on her own, then have Maya appear, them and Sensei spending time together and the disappearance if said disappearance happened almost five years ago.
Still, it's something to consider.
Not necessarily. The accident isn't exactly 8 years ago. Niki said last 8 years:
and recently seemed to compare it to almost a decade:
So, It's likely close to 9 years ago. (It'd explain why Ami was apparently seven):
We don't have an actual date for the accident, even if I do think it's likely the day they visit the graves, or a date where he actually ghosted Niki.
Then there's how Noriko has likely recently became 16, and could have been close to 7 when she was actually tutored. Still, technically 6 though.
We aren't given specific dates for the tutoring (like most things), so the time period is hard to place.
We're also not explicitly told Sensei was a tutor only after the accident or after leaving Niki, as far as I can recall, so he could have started tutoring Noriko just before the accident happened, then resumed it after a break. Based off how Niki was the one who told her family about Sensei's tutoring:
It seems to be a thing he was doing before he started ghosting her.
Also Noriko didn't really need to be accepted back into Sensei's life. Like she said:
In theory, the accident actually brought them closer together soon after it separated him from her sister. Then a short time later, Maya showed up.
Of course, this is all estimation. The actual timeline for these events is purposely murky, and a lot happened.