At 40-year-old Mount Hebron High School in Howard County, Md., the hallways are cramped, the walls are cracked and wastewater has backed up in classrooms. But tearing down the school and building anew, school officials say, could cost $90 million. The good old days of six or seven years ago, when a high school in the Washington, D.C., region could be built for $40 million, are long past. Double-digit annual increases in school construction costs have left the price about $240 per square foot for a new building and site development, more than double what it was in 2000. Elementary schools that once could be built for $10 million now cost $20 million, and middle schools carry price tags of $30 million to $35 million.