San Francisco's schoolyards have long been off-limits on the weekends and holidays, most with large padlocks and chain-link fences that keep families out. The city and school district have decided to change that, announcing a pilot program Monday that would allow weekend access at up to 14 school playgrounds.The idea sounds simple enough: Open the gates, let kids in, and then shut the gates at the end of the day. But actually putting that into practice is not so easy.