SÃO PAULO—When a gunman slaughtered 16 schoolchildren in Scotland in 1996, Britain effectively banned handguns. Australia, Canada, and Germany also tightened gun controls in response to their own brutal school massacres over recent decades. New Zealand is vowing tougher measures, too, in response to this week’s massacre in two Christchurch mosques.

But as Brazil reels from one of its worst-ever school shootings, the response from President Jair Bolsonaro’s right-wing administration and his supporters this week has been the...