It is very common for girls to hide their giftedness and even more common for younger siblings to think they are not very clever compared to their older siblings. With DS2 I have always promoted his sportiness and musical ability as he will never be as good at Maths as DS1 but is superior to him in English (not an important subject to a boy!
As for how seriously will your teacher take it this is my experience of various teachers. I will use "some" meaning teachers but I know this word annoys certain posters - no offence is ment:
1) Some will think every child has an IQ above 140 as they do not really understand what it means and not do anything else.
2) Some will only be interested in what your child cannot do and highlight that continuously.
3) Some will be very positive but be honest and say they will try and cater for the child but may not be able to. In my opinion these are the best teachers as they are aware but do not make any promises and often extend your child better than you could have thought.
4) Some will immediately think you are a pushy parent and switch of or become defensive because they feel that you are saying that they haven't been extending your child. Of course neither is true!
5) I'm sure the excellent teachers who post on here may be nothing like the above but this has been my experience.
In answer to your questions I would not tell the teacher initially and try and enrich her and see just what she can do. So give her maths at home that is more difficult than at school and if she can do it then I would approach the teacher once you have been extending her knowledge for a couple of months. If the teacher has not seen an improvement and moved her up without you bringing this to her attention then she/he is either a poor teacher or has too much to deal with in the class or DD is hiding her abilities.
On the bright side it looks as if DD will get into GS but tutoring may need to begin earlier in order to raise her self-esteem and aspirations.