So... if race was as well-defined as gender, it would be okay for teachers to refer to their classes by race the way they refer to them now by "boys and girls"?
I think you're being ridiculous. I don't have a problem with a teacher saying "students" or "children". I also think there's an element of shaming in comparing the term "boys and girls" to something like "blacks, whites, Asians, Latinos, etc." as if it's even problematic to even recognize that boys and girls exist. You could've used an example such as hair color. Brevity is also important.
The only time I recall gender being brought up in elementary school is when we had to line up and go to the restroom. Putting the kids into lines is just the best way of managing them and the quickest way to get them to organize themselves. Kids aren't really going to hesitate when it comes to grouping themselves as male or female. Even a trans kid is going to group themselves with the group that they are expected to be grouped with, before coming out. But using things like interests makes things more complicated and the kids will have to stop and think about which choice they prefer more.
I didn't say it would hurt anything. Although, now that you mention it, it could come off as rude because people typically present as what they want to be called, when it comes to gender. Usually the name is a giveaway, too. I know I'd be offended if I was asked what my gender is all the time when I'm making an effort to present as male. I do not like being reminded that I am trans since being trans is a painful thing.
With what you wrote previously in mind, please tell me what you mean when you say that they "don't make the effort to present as the gender [they] want people to see [them] as". If the whole spectrum of behaviour and dress can be either entirely male or entirely female, how could a person behave or dress in a way that communicates to you that they're "not male" or "not female"?
That's just common sense. I'm a transsexual man and I want society to recognize me as a man. Like it or not, there are many visual cues that people rely on in order to determine your gender. Much of this is subconscious and evolution plays a role in this.
When transsexuals transition, they're not only medically transitioning, they are also taking on the social identity of a man or a female. That includes presenting in a masculine or feminine manner which makes it easier to read the gender the person is. So trans men will bind our chests to make it appear flat, use prosthetic penises to pack with which creates a male bulge, get masculine haircuts, wear men's clothing, etc. Trans women may tuck their male genitals back in order to create the appearance of a female crotch, grow their hair out, wear women's clothing, etc.
There are also differences to the way males and females walk and differences between how males and females talk (it's much more than just how deep or high your voice is). So many of them under voice therapy and coaching in order to bring those traits into the range of their target gender.
Also, some of this is done simply because it is psychologically gratifying to us to see the traits we want on ourselves. I enjoy the way my chest looks when its bound and hate how it looks when it's not bound. I like the way my groin looks when I have a packer in my under. I enjoy shaving my head because it's a masculine look. Etc.
So we mostly rely on visual and vocal cues to "read" the gender of people. I don't care if you like it or not but that's just how it is.
So when someone is expecting to be called one thing when they're obviously not presenting as that gender, they're just being ridiculous. Those people are also making a mockery out of transsexuals who try so hard to fit into their social role as men and women. Then some girl wearing a push up bra, tank top and mini shirt gets mad because she isn't being called by male pronouns? Excuse me, but that's nonsense. You have to put effort into your presentation. If you don't even bother, than you're not going to be taken seriously and you shouldn't be, either.
I know this may come as a surprise to some, but most trans people are not attention whores and radicals. We mostly want to be left alone, not have to explain ourselves all the time and we're not out to "smash gender" and "destroy the binary".