Don't lie, no matter what.

Buy the suit, and something else that's not so formal that she also loves (even if it's not from the girl's department), and then let her choose on the day of the event. Six-year-olds can't really think that far ahead, you know? Given two equally lovely options, she may choose the less "scandalous" one.

She goes to school, so she's well aware that there are differences between boys and girls and what they wear, even if she can't articulate it. If someone says the wrong thing, she may be embarrassed and the realization that her clothing was not accepted may come down hard. When things are IMPORTANT, as this clearly is to her, the impact of ridicule or the wrong thing said can take on the same gravity and make a longer lasting impression. I think you should do your best to protect her from that, and prepare her for the implications.

If she decides to wear the suit, I would definitely call the teacher and let him/her know in no uncertain terms that there will be no comments made. I think it would be fair to the teacher to get a warning. The teacher may or may not be surprised or shocked, but when we are surprised or shocked we say things which, at the very least, can be misinterpreted.

Ah, parenting...never easy.

Karen