If the child is smart he/she will still be the top of the class, even if pushed ahead.
If the child is not smart they will still be on the bottom of the class, even if left behind.
My Yeshiva has on many occasions picked the valdictorian/speaker to be the youngest of the class, there was no reason to push them off a year, and they were just as capable as children 6 Mo. to a year older.