Revenues from the sale of Proposition S bonds are lower than expected, and San Diego Unified School District officials may have to reshuffle plans for renovating schools, trustees were told Tuesday.The district expects to get about $100 million from the sale of Proposition S bonds for the next academic year, or about $20 million less than projected, said Stuart Markey, the executive director of the district's bond program.Markey expects $20 million to $25 million less in bond revenue in each of the following four years.Voters approved the $2.1 billion bond measure last November."It's trending better," Markey said. "It got worse, but it's trending better."Interest rates and market conditions can affect revenues from bond sales.If the economy improves, revenues could eventually match the projections, he said.The board has been trying to put off prioritizing projects until district staffers finish a study of what the district's demographics will look like in the future, but some of the projects were laid out in the proposition itself.On Tuesday evening, board members are set to vote on an agreement with unions regarding the use of unionized labor on Proposition S projects.Backers of the deal believe it will save money, because it ensures projects will be finished on time, and that San Diego-area union workers will benefit from the government projects.Opponents say the deal is official favoritism, since nonunion construction firms are not being allowed to bid on the jobs. They also say requiring union-only contractors will drive costs beyond what the bond measure is bringing in and, ultimately, fewer projects will be completed.