I saw this happened in the afternoon too. If anyone can show me how to upload photos, I can show photos.
I saw it on 6 Sept 2011 around 13:30. Two teachers led two groups of young pupils respectively (each group is about 10 and they looked very small so I guess they could be as young as k1) coming out from the side door of the school and turning into the back door in the back street near the park, as merci_k mentioned.
Then I saw a few school bus pupils got taken to the back door from the school bus. And this time, I took photos.
Would any parent know the reason behind this arrangement?