Idaho school progress stagnated last year, with only 62 percent of schools again meeting targets.     Superintendent of Public Instruction Tom Luna said he was pleased that overall school performance mirrored the previous year's figure, given that tight budgets could have led to even poorer math and reading test results. Of 663 schools tested, 410 achieved No Child Left Behind's adequate yearly progress benchmarks.  Even so, Idaho decided last week it wouldn't boost the standards, as had been foreseen by the federal law. If it had, another 125 schools wouldn't have made the benchmarks and would have been under the gun to improve - or else eventually face forced restructuring.