Well... I'd say exactly the same thing as you do :"Mind your own business!"

I think it's more the "social pressure" and preformatted thinking that is in question here as some people consider that sucking the thumb is a "baby thing" and that being 8 yo is way to old for doing that.

But before going directly to n+1 (the assistant princpal), I'd personally ask for a meeting with the teacher first and then with the PE coach ("PE" = Physical Education = Sports coach ?) separately (never good to have 2 in front of you, it's not a discussion anymore but quickly becomes a confrontation) and try to understand what is bugging them with your daughter sucking her thumb.

If clearly you can't make them understand your point of view (and that's the only one really important here) then contact the assistant principal and explain YOUR problem of communication with your DD's teachers.

Of course that's only my point of view.