Hi nursiecat:
I think chipping and pointy may be right. You do need to raise this with the teacher and I see that you are going to see your DDs teacher soon.
What I would say is that schools are obliged to provide for 'gifted' children. IMO if a school is using a child to teach other children it isn't any good for the child and it is rather lazy teaching.
Given what is now happening, I would go through with the teacher why she feels a 'chill' year and helping struggling learners will be of educational benefit to your daughter. Don't be fobbed off with this is good for her socially answers. The question is how is this good for her educational development.
The government's own handout on homework and what should happen at school says: 'If children are working a long way above or below the expected level, the school must provide extra support.' You can download the document here - quote is from So what happens at school? section www.direct.gov.uk/en/Parents/Schoolslearninganddevelopment/HelpingYourChildToLearn/DG_185973
I am afraid that if the teacher isn't particularly helpful, I would recommend you raise this with the head. What I will say is that some schools move children working at a higher level up a year for certain subjects. So a child otherwise working at Y3, but working Y4 in maths, would join Y4 everyday for their maths lesson. It would simply require that both classes offer maths at approximately the same time. The other possibility is that the forms at your DDs school may be differentiated and perhaps she should be moved to a more challenging form.
If no joy, follow procedure. Talk to head. If nothing is done, then I suggest you formally complain to governors and then LEA. This is essential because this is visible to OFSTED - and surprisingly enough helps concentrates minds.
Every child is entitled to an education and that includes your daughter.