Are you sure that her progress is down to her making more effort than absolutely everyone else in the class? Could it be that she just had an off week last year and a good week this year? Could it be that she just didn't gel with last year's teacher and has with you? Could it be that the year 5 tests do not give an accurate reflection of her ability? Had she coasted for a year or two and so is just 'catching up' now? Therefore should she get a reward at all?