Just out of curiosity, in what place and/or era is this girl living? In modern times, even if her mother's reticent to talk (or not available), a girl might hit the internet - not an infallible source of information itself, but a place most modern kids would think to look. Depending on the community, the local library might be an option. Isolated and/or very conservative areas may have a religious-based (often mis-)education, with emphasis on original sin and Eve's mistake and so forth. Then you get into various countries and cultures with different takes on the matter; IIRC, some rural areas in India still require menstruating women to live alone and not "stain" the house or other people while bleeding, and it was only a year or so ago (again, IIRC) that a woman ran a marathon without a pad during her period to address the stigma attached to menstruation around the world and specifically in her home country (may have been Africa or Asian.)
The book many girls encountered periods in was, IIRC, Are You There, God? It's Me, Margaret, by Judy Blume. (Menstruation also plays a role as a womanhood rite-of-passage in the MG sci-fi tale Saturday, The Twelfth of October, by Norma Fox Mazer, about a girl who travels back in time to meet prehistoric humans. And then there's Stephen King's Carrie...)
As for my own experience, mostly I remember fifth grade health class talking about it, with the usual "girls watch the one video, boys go elsewhere to watch another" setup, and a school nurse answering questions in a rather bored fashion. (She was an older woman, and still seemed to consider the old midcentury bulky pad with the belt strap a viable option when it was rather obsolete.) I also remember hoping against hope I wouldn't get one - I never wanted to be a mom anyway, so it seemed like a pointless and messy complication - and being hacked off as heck when it finally arrived, round about seventh grade. (Middle of the school day, too - didn't realize until I got home. Very hacked off, embarrassed, etc.) It wasn't a big Thing in our house; no "celebration of womanhood" or anything. Mostly an inconvenience, though not as bad as some relatives got - two have (or had; one's postmenopausal) a tendency to debilitating cramps, and one has heavy bleeding issues that exceed capacity of most hygiene products.
As for misconceptions, I didn't have much of a social circle, and gossiping friends and friends-of-friends tended to be the big sources of info as a kid (pre-internet days), so I can't say I had much thought at all on the matter of menstruation. One was more likely to hear giggly, generally inaccurate things about sex than about periods, but that was just my experience.
Oh, and if you're looking for older "educational" matter on periods and don't mind some laughs, Rifftrax has a few riff-augmented vintage shorts available.