I went to very good schools and am considered quite smart. There is no way I could teach maths, physics, chemistry, geography, French, Spanish, Latin, economics or the woodworking, metalworking etc that I was taught at school.
I still know a bit about all the things I was taught but nowhere near enough to teach a kid to the level required to pass O or A level exams.
Also I had 9 different teachers at school, one for each subject.