I had a lot of conversations around this throughout secondary, my main message was 'You may find X suits you, you might not in a month, that's normal. Part of puberty is all of that is still developing, for some all this is still developing well into adulthood - no rush now"
Due to how these things are taught in school and framed in the media (a rant for another time) I soon added on "You may experience pressure from others to claim some identity publicly, people may try to put a label on you. That is nothing to do with you, that's to make them comfortable. You are not less you, not less 'proud', keeping your developing feelings private. Once you give the information to another, it's out of your hands, you lose control on it, and I don't advise giving it in any situation where you are not free to walk away, which you can't in school."
We discussed crushes as the intense feeling of wanting to be close to someone, not something inherently sexual, and that some people get these at very young age (which is why I'm very clear with mine that crushes aren't automatically sexual), some get crushes around puberty, some start having crushes when they're adults, some people get crushes easily both on people they know and strangers like those they see on TV, some get crushes rarely and only with a close friend, some people never get crushes - doesn't mean they're asexual, some people just have more mellow emotions.
We've had so, so many conversations on different ideas around gender and gender identity, it's ever changing and I'm not less confident on it, but I do remember for the 'can't use changing' sadness, my now 19 year old said exactly that at 12, I said that quite a few people feel that way. Then I asked how she would feel if any guy she knew was changing in the swimming bath locker room and we talked through the feelings. I then showed her a picture of the male changing rooms at the same place which were published empty when they were refurbished - their dividers were only waist high. I asked if she thought any girl she knew would be safe or comfortable changing in there, whatever identity the girl had. We discussed how a lot of changing and toilet facilities are terribly designed for safety, they are the out of the way, leftover spaces with little thought on how to get safety in an emergency. Some places are trying to build safer versions, and we shifted into discussing how could we make spaces safer rather than making it about gender identity. I've found a lot of things around sexuality and even more gender identity can be discussed from another social angle.