At least 21 teenagers, the youngest possibly just 13, died at the weekend after a night out at a township tavern in South Africa, in a tragedy whose cause remains unclear. Many are thought to have been students celebrating the end of their high-school exams on Saturday night, provincial officials said. There were no visible wounds on the bodies. Officials have ruled out a stampede as a possible cause and said autopsies would determine if the deaths could be linked to poisoning. Crowds of people, including parents whose children were missing, gathered on Sunday outside the tavern where the trag…