Surprised not to see any discussion here of Khanmigo[0], which I believe has been using GPT-4 as a tutor for quite a while now (in a beta form). It's been long enough that I've actually been (idly) trying to find any efficacy data. I'm sure that by now Khan academy has it, but I haven't seen them release it anywhere.
The famous tutoring 2-sigma result (referenced elsewhere in the comments), only took place over 6 weeks of learning, and Khanmigo should have over 6 months (I believe) of data by this point
I imagine they're doing a lot of development and it can be hard to make sure new products are eg gdpr compatible (not to mention the fact that they might want to have it do different things in order to help students in other countries)
The famous tutoring 2-sigma result (referenced elsewhere in the comments), only took place over 6 weeks of learning, and Khanmigo should have over 6 months (I believe) of data by this point
[0]https://www.khanacademy.org/khan-labs