You're quite right Brownie, I hadn't even thought, just automatically assume short hair = boy!
Surely if DC is being mocked every day, that constitutes bullying, which is a matter the teacher should be involved in? If a DC was bullied for, say, wearing a crucifix, would you tell them to remove it?
Clearly don't just tell the child to put up with teasing, get the teasing stopped.
I would think of this as a societal beauty standard in the making or in transition - it can't be that entrenched, at six. It may also only be tangentially linked here to adult beauty standards - kids may just doing an inconsequential us-and-them thing, noticing it as 'you have this and we don't', which is backed up by vague notions re: what adults' eyebrows look like - but of course the fact that we as teens and adults are forced to make decisions about eyebrows (not shaping is a decision like choosing to shape is) coincidentally is of course very much a direct result of beauty standards, so it's good to try and nip it in the bud before the connection becomes obvious.
E.g. if a little kid had a turned up nose and someone got teased for that, you would think it's just kids exploring difference, not that they had some idea of what noses are more accepted than others, because of course we like turned up noses. Just because, in this case, it does seem to reflect a beauty standard the DC will encounter later doesn't mean that we have to treat it within the adult context - i.e. assume it's a steadfast reality and there's nothing we can do to stop the teasing because it's part of a bigger structure that we can't do anything about. If that makes sense?