By Matthew Freiberg, Greening Urban Watersheds Program Manager After the final bell on a foggy Friday last month, students, teachers, parents, and staff of the Alice Fong Yu School kicked off their weekend with a celebration of the school's new rainwater harvesting system. And the weather couldn't have been more appropriate. About a half hour before the event was to start, the skies opened up and gave the system its first good soaking, sending raindrops from the school's gardenshed roof through a pipe and into a large storage cistern. Then, as if on cue, the rain stopped just as school …