Luckily for us, that is not our situation right now. But if I did have to make that decision today, I wouldn't do it. I think girls can be just as competitive as boys, at least, mine appear to be! The two girls that are intellectually matched and in class with DD8 are EXTREMELY competitive. Not sure where it comes from, but the three are friends and seem to naturally balance their personal need to be the best and the fastest! Not so with DD8 and DD5. My goodness, they will actually hip check each other running upstairs to the bathroom at night to get there first. We absolutely are not encouraging this.
We are in a large school district, there are 4 or 5 junior highs and 2 high schools so perhaps someone in this situation could send the child to another school in the district. That might solve the problem. I do know that younger daughter is more interpersonally talented than big sis. Moving her into the same grade in the same school would be a nightmare for big sis.
Maybe my view would be different, though, if I were actually in this situation.