This is being visibly bored. It drives ranters wild.
They want you to agree or to be upset. They can not handle the fact that it is just washing over you.
You begin to yawn. Examine your finger nails. Count your teeth with your tongue. Ever now and then look back at the speaker and then glance at another student and raise your eyebrows, "Is she STILL talking?" Nod in answer. Yawn some more. Look at the clock.
It is also something that they can not nail you for, unless you are in the military.
Look out the window and gasp.
It will distract everyone.