Secondary schools generally have more spare class rooms and great space for expansion. In addition Sydenham girls is currently being rebuilt, so presumably taking account of increased demand, and Forest Hill Boys was rebuilt less than 10 years ago, with lots of spare room on the site of the old building. Although I think I remember hearing that they might be taking over the Brent Knoll site across the road when it becomes vacant.

And the local council cannot mis-calculating demand for secondary places, the primary schools demonstrate the likely intake with 7 years notice.