Water, Water, everywhere. I can't collect it.

So, I am making some great headway on my gardening project. My downfall....irrigation. The rainwater that falls on my school's roof is immediately divereted (within the walls of the school) to our city storm sewer system. There are no areas that I can find where I could install a rain barrel. Does anyone out there have any suggestions on how I could collect and save some of the water? I could temporarily intsall some old gutters on my shed and collect the water at home and transfer it, but that is a lot of back and forth. Let me know your thoughts please!