Maybe you could start by asking them for specifics?

How do you identify a GT child?
What characteristics do you look for?
What kind of performance are you looking for?
What are your published guidelines for acceptance into GT programs?
What are your specific concerns that DS and DD are not performing as a GT child should.

There could be a whole bunch of reasons why the school is not able to recongnize their talents. I'd see what you are dealing with before you consider that both of your children's tests scores are innacurate.

Good luck,

Incog