I recall that in my home state, the school system there reported that of the myriad of special concessions requests made and given for testing in the final year of school, the majority (I believe over 60%?) came from Private Schools and to the wealthiest areas. (Despite being the minority of schools).
The reason likely being twofold - Access/Knowledge of the system, as well as a pressure to improve their marks.
I.e. These private schools make their name and are in many ways partially paid for by parents because it promises better marks than sending them to public schools.
So they use every trick they have to get special dispensations and raise student's marks, leading to the bulk of these extra marks and time going to the wealthier, privileged areas.
As well as that, note that I'm not saying that all of these students are necessarily cheating by doing this, many of these students have their own problems (though many didn't too!) but the access and knowledge of how to use the system is also a part of being privileged.