First time I heard about it in school was when I was in 4th grade and I learned girl's can get their periods young as nine and I was ten and hadn't gotten it yet. We also learned about tampons. But the way the school explained it, they made it sound like getting your period is painful and all and it freaked half of the girls out in my class, even me. Then a girl in another class across from my classroom got her first period. Surprisenly, I forgot all about it and was reminded about it two years later when I had a bloody accident in music class.
In fifth grade, my mother didn't have me see the videos which was call human growth development so instead I went to the library and had to read about sports. But at the end of sixth grade, they showed the videos about changes in your body and they had the boys and girls separated.