"All right! Everyone in your seats! No more talking!"
This was a standard approach when i was in school, low these many hundreds of years ago, and it still is, to some degree.

*she'll occasionally go into her "teacher voice"*
It comes with the territory I'm afraid, i should know, i am a retired school teacher!

"I say, you, yes you in the back row, Jones minor, sit up and pay attention, and STOP picking your nose!"