The children all seem to be girls, but were there girls' schools on such a scale in those days?
I'd wondered about an orphanage - single-sex, or girls' wing? The portico is fairly similar to the one that survives from the London Orphan Asylum in Clapton. There are plenty of pictures online of this (view from the west), and it now forms part of the Clapton Girls' Academy.
Old maps seem to show what might be a similar portico on the other side of the building, facing Almack Road, but this no longer exists; I'm wondering if this could be what's in our photo, but I haven't found any pictures of that side.