Devastating arguments. I guess I was wrong.
You bring up an important point- how do we know that it's not just a phase? Well, one thing we know for sure at this point is that the earlier true trans kids transition, the fewer problems they have later on, which is what I was getting at with my mention of research, so it's important to take the child's claims seriously, even if they are young. Now that doesn't mean any gender-nonconformity displayed at that age should be treated like a sign of trans identity, and that's an important distinction to make. A boy wanting to try on his mother's jewelry once or twice doesn't mean he's going to be trans, or even generally effeminate. But a boy persistently claiming that he feels like a girl over a longer period of time might be. The research we have now suggests that gender identity is well-formed enough between the ages of 2 and 4 that a boy or girl repeatedly claiming to feel like the other gender at that age will probably be trans, and the percentage of kids at that age for whom it ends up being a phase is very low (in the single digits, I believe- I'll try and find the cite when I have time,) so the time needed to determine it isn't on the order of several years, but less.
Usually, it's best to consult with a psych, even better if it's someone who specializes in gender identity issues. I would tend to trust their diagnoses.
Yes, they are kids. But as I said, all the research we have currently states that human gender identity is well-formed enough between the ages of 2 and 4 to define them for the rest of their lives. See my response to onedeen for that and how to determine who has a gender identity issue. And also remember, since you brought up sexuality, that trans identity and gender identity in general have nothing to do with sexuality- they aren't sexual orientations, but gender identities, so this isn't a matter of forcing kids into sexuality too young. We all knew how we felt in terms of gender long before we knew about or were thinking about sex and sexual orientations.
As for boys sneaking into the girls' locker room- people don't mysteriously become trans as teenagers, without a longer history of gender non-conforming behavior, so that's one way to tell a person is lying. The other is simply that they would have to present like a girl all the time, not just when it came to the locker room. How many non-trans men do you know who would be willing to act like and call themselves a girl all the time just to get a few minutes every week in a girls' locker room, especially in a viciously judgmental middle or high school environment? That's not a trade off I or any of the other dudes I knew back then would make. The social repercussions to presenting yourself as the opposite gender are serious- you get treated like shyt. People generally aren't going to publicly identify as the opposite gender unless it's absolutely essential to their sense of self.
And lastly, my comment about folk-mythology is relevant. I'm not saying that people whose sex and gender match up are misinformed- why would I do that? Examples of folk mythology in this case would be people saying things like "a boy is a boy and a girl is a girl and that's that"- nature just isn't that simple. We can't live in denial as a society when all the evidence is in front of us that there are some people who really feel like the opposite, and that it isn't a mental illness (as illness is scientifically defined.)
In my opinion it opens up too many "what if's" a person can claim they identify with the other gender and not show many overt outside characteristics. It's discriminative to tell someone they are not gender opposite enough to warrant their use of the girls bathroom.
What about the people who go back and forth. They don't have to be lying, they could just be confused. Do we let them switch bathrooms back in forth until they make up their mind? If yes then that allows room for people to lie just to get in the opposite bathroom then change back once they are done. If no then that is discriminative against the people who might of taken longer in their life to realize what they really wanted to do.
It's a bunch of fukkary that shouldn't be going on in schools. If they want to be friendly to transgendered kids they should open a gender neutral bathroom. Kids could choose to use it on their own rather they'd like to use the bathroom and they wouldn't have to declare their sexuality/orientation.