Manic phase can sometime express as extreme irritability, so isn't always obvious as mania.
My DD8 has had the occasional episode of "maybe I should just die," and "I'm bad at everything," which she self-identifies as low self esteem. Any yeah, it's not clear to me whether it's attention-seeking or if she really believes it. We've been fortunate that the episodes rarely last more than a week or two, and then we get a long break. She's over-the-top remorseful and guilty over any misdeeds even then, though, which I see as related. I don't know if it's that she's holding things together but can't do it all the time, or what.
This is a hard age. I'd hoped I'd have a few more years before we got here.
Frankly, I do find it odd (though it's a good thing for us all) that she is so well-behaved at school.
Having had mental health issues as a teen, I don't find it odd at all. I was actively attempting self-harm before I had the slightest bit of bad behavior at school, and that was years after I started acting out at home. Not that your DD is headed down that path, but "fine at school" is not an indication that nothing's wrong. School, even boring too-easy school where you have no friends, has a lot of elements that help you keep it together. Structure, peer pressure, distractions you can lose yourself in.
