@beingamumishard it isn't about the sex, it is about the relationship and it aping an adult relationship where they get to sleep together in the same bed. This girl is in year 10, sleeping over encourages more sex, not less. She has just turned 15. I completely agree with @fourelementary post.
I keep saying this on here, when Ds was in year 10 a girl got pregnant, declared it was the best thing to ever happen to her and kept the baby. The Dad was a stunned year 9 boy, their birthdays were only months apart. Every cocky loud mouth boy in year 10 who was boasting about being sexually active suddenly realised they had no control over whether they became a Dad before getting their GCSEs.
The OP has a DD, have they even discussed pregnancy? STIs? Most parents just cover consent and contraception. Rarely does anyone address the failure of said contraception and considering how many grown women on here post about having an unplanned pregnancy, teens are also going to fuck it up too.
@IamnotSethRogan there is always this suggestion that no sleepover means sex in park or field. I was having sex at 16 at my boyfriend's house and no sleepovers. Today, with cameras on phones and instant sharing I think a lot of girls would think twice about being somewhere they could not only be found, but filmed, shared, shamed, labelled.