Fewer Michigan students are taking optional rigorous high school classes intended to prepare them for college success at a time when Gov. Jennifer Granholm is urging them to take tougher classes. A report released today by ACT, a national college testing service, found that 53 percent of this year's Michigan graduates who took the ACT took a full load of college prep courses - 3 percent less than the national average of 56 percent and a 1 percentage point drop for Michigan students for the third consecutive year. Granholm has proposed that the state provide monetary incentives beginning in 2007 for high schools that adopt more rigorous academic programs, said Liz Boyd, the governor's spokeswoman. A dollar amount has not yet been attached to the incentive program.