With high school kids assuming they are making characteristic sounds use drones and 5ths to get them to hear. The 5th is sometimes easier to hear than unison. F around the room, pairs, trios, etc. are good ways to break up the monotany. In tone, In Tune was a great phrase I picked up from a Saucedo clinic. If the kids don't have the same tone then tuning is a struggle. Pair them off by instrument and have them work in a practice room or in the band hall sounding the same then sounding in tune. If they are very young, start with a tuner but get them off of it quickly. Observed a great 6A band program years ago and they don't have a tuner anywhere in the band hall. But they do at the junior highs so keep that in mind as to where they are and where you need to meet them.

Also, teach the kids how to tune problem notes. If the embouchure is correct and the horn is quality most of it should be pretty close. Learn the bad notes to see some faster gains.