Rachel Dratch was apparently as puzzled as we were that Lexington High School’s graduation ceremony Sunday was held at the Tsongas Arena in Lowell. “Ahhh, Lowell. The air is thick with memories here,” wisecracked the former “Saturday Night Live” comedian, addressing the eager grads. “Remember how we all used to hang out after school by the old textile mill? Those were some good times. Or on weekends, meeting friends at the smelting factory? Awesome! And of course, our senior prom at the die-cutting and mechanical bits manufacturing plant. Really special.” Dratch, who graduated from Lexington High in 2000, was cheered on by her mother, Elaine.