I would try and have a quiet, confidential word with the teacher.
One of two things could happen:
a) They turn out to be unaware of it, and as a result start looking into it (from here, it could turn out to be neglect or similar, or the school could themselves find out that there is a medical problem which is being dealt with, or something else that is equally unconcerning)
b) They are well aware of it and know that either it is nothing to worry about (she's already seeing a GP for example) or that SS are already involved.
Either way, if you say nothing, school are unaware, and then something serious turns out to be going on, the same people who are now saying 'say nothing, it's interfering' will be wondering where all the adults were when this child was in danger. Best to be told that it's all in hand than to say nothing and leave a child in what could be a neglectful, or worse, situation.
Doormat - I agree with you in that it doesn't necessarily indicate abuse or neglect and making those assumptions is maybe a step too far. BUT, I also think that assuming everything is fine is not a good thing - it may be, but if it's not and you do nothing is that not worse? The implications for the child if people assume she's fine and she's not are huge.
At the very least, if the child has hygiene problems, her parents are aware and do their best to get her sorted and clean first thing, they may not be aware that her hygiene deteriorates during the day and a talk with school could mean they put in a plan to change her undies at lunchtime or something.
The other thing I can see as a problem is that it might not be enough to tell the kids not to make nasty comments - if one child smells the others are often reluctant to play with them and this girl could end up very isolated. Again, school may not notice (Year 6 - I don't know how closely they'd be watched at lunchtime for example) - may just lead to school taking more notice for a bit and then deciding for themselves.
You've got nothing to lose by saying something, and the child has everything to lose if you don't.