If we're talking about synchronous classes, you really don't need a specialized platform. I started my business with a squarespace website and a zoom pro account. Zoom's range of screen sharing options are extremely teacher-friendly, especially if you've got an iPad to present from.
I looked into different specialized platforms and one that did stand out is TutorCast, though I think their pricing was unreasonable and prohibitive for a new, small company.
Another model to look at is something like Verbling. It's like a marketplace on which teachers can set up a profile, charge whatever they want, and Verbling just takes a cut of that. I don't love this model but it's clearly working for a lot of teachers.
Hope this helps!