A couple of weeks ago, a gaggle of freshmen at McCallum High School in Austin, Texas, pulled out a pouch the size of a clementine and began batting it back and forth between their feet. “It took a minute for me to realize, Wait, this is hacky sack,” said Sondra Primeaux, 56, a teacher at McCallum. “I haven’t seen this in a while.” Now she sees it constantly. Students circle up with a hacky sack during lunchtime or get in a few kicks in the hallways after class. Primeaux has been having flashbacks to the Phish and Grateful Dead shows of her youth. “One of the boys was like, ‘Where can I get one?’” she recalled. “I said, ‘1992.’” Once the domain of mellow Gen Xers in the ’80s and ’90s, the game is experiencing a renaissance at the hands — well, the feet — of Gen Z. High school students around the country are freshly enthusiastic about the crocheted beanbags that once hung in the air like the scent of marijuana.…