Looking for a story

A few years ago, I read part of a story (I don't think it was on BC but I could be wrong) in which a boy and girl were in a school (I think) library, talking. The girl suddenly ran off, leaving the boy wondering what had happened. He followed the girl who had run into the toilets. He wnet in after her and asked what was wrong, I think she was reluctant, at first, to tell him why she had run off the way she did. eventually, though, she did tell him that she'd had a period (first?) and was embarrassed about it. She also needed clean clothes as what she was wearing was blood stained. The boy offered to go and get her some. That's all I can remember, which is not a lot on which to go, but if anyone recognises it, I'd appreciate it if they could let me know where I can find it. Thank you.

Regards,

Dave.