This is not good - do you know for a fact that this woman is right? Sometimes I hear the most ridiculous stories and they are totally inaccurate. I even had a woman argue with me that Eton took girls! It didn't matter that I'd been there, read the reports etc... she refused to back down.
Sometimes people hear things on the grape-vine - lets hope this is the case.
If it is the case I would be concerned that the teachers were over-stretched or they had to get a high number of new teachers in who didn't know the workings/ethos of the school as well as the others making the year a learning experience for all. Would you consider keeping your child at the prep school until 13 and then seeing how well the school coped with the large year group and think about sending him at 13 if there were no negative effects? Then you could keep your options open.