This month's four-day teacher strike, which mercifully concluded this morning, should not have caught anyone off guard. The San Francisco Unified School District and its teachers had been at loggerheads since March of last year. On Oct. 10, the two sides formally declared an impasse and ceased negotiating. In November, teachers held "practice pickets" at more than 100 schools. In early December they voted to authorize a strike by a 99 percent clip and then, in January, did so again by a nearly 98 percent