The Urban Myth of College Drop-outs

Recently another pastor friend and I have been trying to figure where the most popular statistic in youth ministry comes from. The statistic is, "85% of all youth drop out of church when they graduate high school." Does anyone truly know where this statistic comes from? I have done some research and everything seems to point to an unknown source. Does anyone have any hard evidence? If you have any info please share it with the rest of us.