"Teacher" seems to be a flexible term here.
There's a big difference between someone who's studied and practiced teaching a given subject and explaining it in a way that kids will understand, and a parent who knows only what they have read in the textbook.
And in a school, it's not "1 teacher" - a kid will have a team of teachers, each with their own speciality. I think my English teacher would have done a pretty dodgy job if she'd had to teach us chemistry or biology.