"was there ever an explanation of why Larry Mondello left?"
Not that I know of. Gilbert and Whitey kind of came and went too.
"wasn't Miss Landers Beaver's teacher for at least two or three years? "
Yes. Also, earlier he had Miss Canfield. And there were a couple of teachers (men - Mr Blair and another I can't recall at the moment) who were specifically History or English teachers.
LOL.
A Leave it to Beaver historian.
You need help that no vegetables, even cooked in bacon fat, can provide.