Washington (AFP) – Fourteen students and a teacher were shot dead Tuesday when an 18-year-old gunman opened fire at their Texas elementary school, the latest in the United States’ relentless cycle of school mass shootings. Here are America’s deadliest classroom gun massacres in the last two decades. Columbine High School (1999)Two teenagers from Columbine, Colorado, armed with an assortment of weapons and homemade bombs, went on a rampage at their local high school. Twelve students and a teacher were killed during the April 20 massacre. Another 24 people were wounded. Columbine, whose name has…