I try to have each kid master these certain buzzin criteria before putting it on full instrument:
1. Embouchure/MP placement
2. Breathing through corners of mouth with mouthpiece only
3. Able to buzz a whole note with steady air for 4-8-12 counts
4. Able to tongue properly, start notes with tongue and tongue quarter notes without stopping air
5. Able to buzz and repeat rhythms I buzz to them with tongue
6. Sirens. Students should be able to do a decent sounding siren with a clear change in pitch from low to high and back.
After a week on mouthpieces most of my students can achieve these concepts and it makes putting it on the full instrument much easier. Teaching them to tongue only on the mouthpiece is so much easier than on the full horn.
Also getting them on the right partial early on is so much easier if they can do sirens because they understand how to manipulate their air and embouchure to change the pitch.
Just some thoughts.