From being in an amateur sports club (and also having worked in a School), we see a huge drop off in girls playing sport between 13-16.
That age range coincides with moving up to senior (or young adult age groups) participation from junior, ie when it starts to become less of a 'fun activity'
But also secondary school. As you say some of that is boys, but there also (for boys and girls) the wanting to concentrate on school for some, not wanting to be seen as different by doing something most aren't doing, being forced to pick just one sport by 16 if you are serious about it and (specific to girls) not wanting to be seen as the sporty girl which in the school world is deemed less feminine and therefore different and bad(

School life)
Unfortunately one of the things schools lose first with cut backs is extra school activities (sports etc)