WASHINGTON, Feb. 22 — High school students nationwide are taking seemingly tougher courses and earning better grades, but their reading skills are not improving through the effort, according to two federal reports released here Thursday that cite grade inflation as a possible explanation. The National Assessment of Educational Progress, an exam commonly known as the nation’s report card, found that the reading skills of 12th graders tested in 2005 were significantly worse than those of students in 1992, when a comparable test was first given, and essentially flat since students previously took the exam in 2002. The test results also...