Basically, from what I can tell, (I may not be entirely accurate)
The Cheltenham Girls students were told they were NOT doing the Catholic Trial. It was set for a week before the Catholic Trials should be set. The exam contained elements of the Catholic Trial (or, was the Catholic Trial but modified to look differently, I don't know. More than likely the former though) .. Because of this, if Cheltenham Girls students had spoken to people from other schools doing the Catholic Trials, those people from the other schools could have studied those questions specifically, hence gaining an unfair advantage over those who didn't.
The part I don't understand is, how could the students from other schools have possibly known that the questions from Cheltenham's Trial would've been in theirs anyway?