By Brad Brooks and Gabriella Borter UVALDE, Texas (Reuters) -Panicked children called 911 at least half a dozen times from the Texas classrooms where a massacre was unfolding, pleading for police to intervene, as some 20 officers waited in the hallway nearly an hour before entering and killing the gunman, authorities said on Friday. At least two children called the 911 emergency number from connecting fourth-grade classrooms after 18-year-old Salvador Ramos entered with an AR-15 semi-automatic rifle, according to Colonel Steven McCraw, director of the Texas Department of Public Safety. Ramos k…