SARASOTA, Fla.—People from all stages of life—with and without children in public schools—are filling workshops across the country designed to prepare them to challenge local school boards on issues and help school board candidates they support get elected. In January, it was a mostly conservative crowd, hopeful they’d learn how to right a lop-sided, left-leaning school board, who attended activist training in Sarasota offered by the Leadership Institute. The institute, officially nonpartisan, is based in Arlington, Virginia. Its instructors have mentored people of a variety of beliefs on how to become effectively involved in politics. But the group unabashedly aims to give conservatives an edge. That’s an advantage some Sarasota residents say they desperately need. The beach community, about midway down the western edge of the peninsula of Florida, has 349,049 voters, according to the county’s supervisor of elections. Of those, about 43 percent are registered Republicans, and 30 percent are …