The bombing was quite deliberate. There were Canadian troops billeted in the area around the school and military vehicles (fire engines) were parked in the school playground. This probably convinced the German Intelligence that it was actually a military target. It was the only school in London bombed during daylight. If schools were bombed during the night the children would not have been there. The Germans stated after the raid that they had bombed their target.
It is curious that the aeroplanes were able to penetrate so far inland. But I suspect that the authorities were not prepared for the unexpected.