The sleek Juul electronic cigarettes have become a phenomenon at U.S. high schools, vexing educators and drawing regulatory scrutiny over their sweet flavors and high nicotine content.Now, a new wave of lower-priced Juul knock-offs is showing up at convenience stores, vape shops and online - despite a U.S. Food and Drug Administration rule banning the sale of new e-cigarette products after August 2016 without regulatory approval.