Hi Hazel,
I have a son who is T1 & in P6. If your daughters reading ability at home hasn't gone down in your opinion i would certainly ask for a re test. Could be that your daughter was having an off day when tested or if her blood sugars where running a little high/low her concentration may have been affected. Its worth asking her teacher if she has taken into account her reading ability over the last few months and not over a single test. I know how anxious these tests are at this stage of our childrens education, as if they haven't already got enough to deal with.
Let us know how you get on and good luck,
Kind regards,
Suzi x