I say let her wear it. Trying to convince her to wear something else now that she's picked it out and seems to love her choice will likely crush her more than anything her peers might say. She values your opinion much more than anyone else's

The adults won't say anything to her that is negative. They may whisper behind her back and make sure YOU hear it (you know the type of parent I'm talking about), but they won't say anything to her face that she would perceive as negative.

The kids in her class may say something. Most likely that will be "why are you wearing a boy's suit?" to which she should have a ready answer ("because I like it"). I'd think the vast majority are just going to be curious--kindergarten kids of course understand gender differences (and all kinds of other differences) but are also remarkably open minded and accepting. The only exception will be those kids whose minds have been poisoned by close minded parents. I suspect that if there are kids in your daughter's class like that, she's already told you about them and is already used to dealing with them, so she may want to prepare an answer for them too. By the same token, they already know her and know that she's not a frilly dressy girl, so in all likelihood it won't be an issue. If this were her first day of school I might be more hesitant about it.

And the most likely scenario is that the adults in the school will think she's the most adorable thing they've ever seen--it's refreshing to see a kid who is just herself and doesn't care what other people think of her when the rest of the school is dressing up like Hannah Montana.

Good luck!

Sarah