Gwinnett, the state’s largest school district, saw 610 employees retire in 2012, the school board was told Thursday. The biggest single category of teachers. More than 350 of them left. Teachers had an incentive to leave in 2012. An extra pension benefit that put more money in teachers’ pockets was phased out for 2013 and beyond. Also leaving the school system were: seven principals; 18 assistant principals; 53 paraprofessionals; 50 clerical workers; 38 school nutrition workers; and 16 custodians, according to a report given to the school board.