If you're writing letters - you could put in a request to see her educational and curricular records as well; you might find something interesting there as well.
Link to primary maths framework have a look through see what you think she can/can't do, ask the school for written evidence that she can do enough to achieve a 2b!
nationalstrategies.standards.dcsf.gov.uk/node/19964
We also had a maths tutor straight after Yr2 SAT results - gave her SAT results plus reports from Ed Psych and info re what I saw as his difficulties etc. The first week she said 'we'll have him up to speed in no time, don't worry'. Two weeks later I asked her if she'd thought I'd been exagerrating!
She explained that in some schools teachers are coerced into signing off that a child can do things they can't.
Unfortunately this does have a knock on effect as the next key stage, even when it's a different school; they don't want to say we received this child as a X and she has made no progress since she's been with us, but it's also difficult for them to 'whistleblow' on the previous school; probably as so many do it.
The first secondary my son attended I
was told by the SENCO that the school had 10 children who couldn't have achieved the levels they alledgedly gained at primary - L3/4's, though 2 couldn't even read; the school had to put in an intensive english group for those 10 (one my child), yet she still sat in a meeting immediately after telling me this and didn't say a word, while the head of english was telling me my son doesn't have any probs in english!
Current head has also admitted that they get a number of children each year from certain schools who couldn't have achieved KS2 results - they do nothing.
Schools still trying to deny difficulties - 2 end of module maths tests recently, he sat in the corridor with a TA helping with all the answers! Which according to the head is acceptable if it helps child achieve
You will have a battle on your hands to get them to admit they are wrong, but it's probably better to upset/antagonise the current school rather than have to immediately go head to head with the new one