It's the time of year when talk of snow days creeps back into our schoolday conversations. But even before the first snowflake fell, cold temperatures have forced the cancellation of classes at a handful of Memphis schools. The city has more than its share of aging school buildings, and four state-run schools closed yesterday due to heating problems. Two others operated by Shelby County Schools dismissed students early over the same issue. And other facility problems have kept students home this semester too, exposing the bigger issue of costly overdue repairs and maintenance.