I'm scared that they ask tall about and dwell on these things that they will actually manifest (depression etc, but sexuality obviously)
I'm not quite sure I'm reading this correctly (this isn't me being a pedant about typos, just that it is relevant to say I might have misunderstood). But do you mean you're scared she will dwell on issues relating to depression/sexuality until those issues manifest?
If so, I think I feel a bit uncomfortable with that idea. Someone doesn't become bisexual or a lesbian because they spent a lot of time thinking about it aged 12. However, a 12 year old who is bisexual or a lesbian might find it quite helpful if it's socially acceptable for lots of their peers to think lots about sexuality.
I wasn't born yesterday and get that teenager girls do often go through the 'trendy sexuality' phase. I don't think it is harmful, though, and surely it is so, so much healthier than what happened when we were young, which was that you weren't really allowed to talk about it?
I can't help noticing the first response from @AllTheWhoresOfMalta mentions that the only lesbian she knew at school was 'quiet' about it and didn't come out until university. Isn't that actually rather sad? While most of her classmates were enjoying flirting and maybe first dates and so on, she presumably missed out on all of those teenage experiences.
That's actually really common for young women who aren't straight. And you'll never get it back. If you feel unable to talk about your sexuality until you're 18 or 20, you are never going to get to do the discussion of teenage crushes or the excited flirting or anything all of your peers are doing. I don't think we think about that enough.
And if your DD isn't bisexual or a lesbian, where's the harm? I don't believe, in this day and age, that any decent man is going to jilt her at the altar when he finds out she kissed a girl and didn't like it.