<sod >MONTERREY, Mexico -  A middle school student opened fire at an American school in northern Mexico on Wednesday, gravely injuring four students and a teacher, said Nuevo Leon government secretary, Manuel Florentino Gonzalez.< eod>< sod > MONTERREY, Mexico -  A middle school student opened fire at an American school in northern Mexico on Wednesday, gravely injuring three students and a teacher, before turning the gun to himself.< eod>