Well here is yet another reason to think that school uniform is a really really bad idea. If my dd's teacher had told my dd at six that she had to wear shorts underneath her school dress I would have been very angry and told her that she had absolutely no right to say such a thing.
The OP's dd was not wearing a thong, but ordinary six year old pants, which are hardly 'immodest'. If she had been wearing a thong then fair enough for the teacher to have a quiet word with a parent (not the child). Allowing girls who wish to wear shorts or trousers is one thing, but forcing them to cover up because of a teacher's sensibilities is really outrageous.
The only time my dd has worn shorts under a skirt is when the skirt was very short and it was a part of the design of the outfit.
Also whoever said that in the US and Canada school have skorts as a part of their uniform, very very schools out here have uniforms at all because teachers trust children to wear appropriate enough clothes to get by just fine without excessive 'guidance'. Some girls wear very short skirts and some boys wear low slung trousers, and it really really doesn't matter.