what i do that works most of the time:
raise my hand
in a voice slightly lower than my normal level say:
'If you can see my hand and hear my voice, raise your hand and be quiet"
keep repeating steadily until all are quiet and listening
the first ones to get it will pressure the late listeners to respond.
when all are quiet i say "thank you" and begin the lesson in a quiet voice.
sometimes it works-depends on who is in suspension or jail
sometimes it doesn't-full moon, recent fight, pep rally or other
disturbance in the force
best wishes for a great year doing the most important job in the world
nancy in north texas