One of the most effective ways to get a class to quiet down is to stand
somewhere in the room and say "If you can hear me, clap once" (those
within earshot will clap).
Then say, "if you can hear me, clap twice." (more kids will clap along
with the first clappers).
Finally, say, "if you can hear me three times" By this time, ALL of
the students will be clapping and will give you their attention!
Works for me...!