might need extra tuition in school but the other two, non-disadvantaged groups won't.
How about somebody from a non-disadvantaged group who's just thick?
I'm joking, but not entirely. I see where you (and Bion and Raudona) are coming from, but considering that '(dis-)advantaged' is a very vague term* and 'non-disadvantaged' a bit of a linguistic Ungetüm, I do feel that people are going out of their way to avoid any kind of clear reference to the fact that some children do have a better start. It wouldn't make the sentence much longer if you said, for instance,
Researchers found that 56% of disadvantaged pupils achieved a 4 or above in English and maths, compared with 78% of those from neutral or privileged backgrounds.
(That's the origial sentence, only with 'non-disadvantaged', of course.)
*I, for example, wouldn't know in which group to put myself. I come from a working-class background, my brother and I the first generation ever to attend a Gymnasium; there's wasn't a lot of money and my parents were able to help with almost none of my school subjects after primary school because they themselves left school at 14. At least two of my schoolmates in similar situations, though easily bright enough, didn't go to Gymnasium because their parents were too scared of what that involved.
On the other hand, my mum comes from a family of avid readers, so her German is excellent despite very little formal knowlege, and both my brother and I grew up speaking proper German. She also read to us a lot und made us love books, and my parents wanted us to do well in school and always took an interest, making us do our homework, made sure we had all our stuff etc.
I don't know but assume that we would have been classified als 'disadvantaged' but I don't think we were. We certainly didn't feel it.