I agree to have your child say in a neutral tone that the school decided 5th was a better fit, and leave it there. Don't over any more info--it's not their business. If more questions come, she can just say she doesn't know.

My son is frequently asked how old he is, since he's still pre-puberty in high school. He doesn't answer that question. Instead, he just says, "I'm in 9th grade [and then changes the subject.]"