I don’t really know how else to do this other than by putting some awareness of the idea out there.

Kids, just ignore any momentary penalties and conformity pressures you might be feeling and pull out your phone to start recording the teacher without saying anything. Act like nothing is wrong, just blankly ask them with a complacent demeanour to repeat themselves if you missed your chance to record them. Any hard evidence of this is good and the threat of being recorded should dissuade them hopefully.