We do F around the room every day, find the section that is the worse (sometimes 2) and tune that section(s). Also, breath support is really important, can't tune bad air support.
When working music, we'll spot tune certain chords, pitch by pitch because a lot of the times the octaves are worse offenders than 5ths or 3rds. Start with the root, add the 5th, then the 3rd, then any other note. That way the kids hear the notes in order of importance.
We also have a chord tuning exercise we do daily that helps the kids learn to listen to part relationships.