The minute they heard their son was one of the missing normalista students believed to have been killed by police in Mexico, Manuel and Hilda González got on a bus and traveled nine hours to make it here, the Raul Isidro Burgos Ayotzinapa Normal School, in the southern state of Guerrero. Their son, César Manuel González Hernández, is 19 and a second-year teaching student at Ayotzinapa, a Revolutionary-era rural teachers college known nationally for the ardently leftist politics that guide everything the students do and study. César is now among the 43 normalista students missing since September 26, when a...