how do you find an educator and join a class?
"Duolingo for Schools" is for teachers to use with their real classrooms. They're not supposed to be used by the general public because they reveal email addresses. That's no big deal for a teacher who already knows the school email addresses of her students, but for strangers on the Internet it's a different story.
You might find what you're looking for on italki. https://www.italki.com/home