1. Inside voices
Trumpeters might need to be reminded that concert band is not marching band. While a certain volume and brightness is acceptable on the field, and perhaps necessary to project outdoors, this is not acceptable inside. Students will need to play softer in general. Their loudest dynamic inside will be less than the same outside. Students should strive for a warmer sound.
For concert band, I think a good rule of thumb is to have section members play a little softer than the first chair player of each part; make sure they can hear the first chair player while playing (suggestion from Walter Chesnut). Make sure they understand which part is most important at various parts in the music. Make sure they can hear that part (if it is not theirs) while they are playing. Sometimes, all parts within the section are equal; ask them to make sure they can hear all parts equally. For practice, rotate the players so they all get experience playing every part, and therefore get familiar with every part. Often, it is the inner parts that need to play out more. Exercises to practice this can be helpful. For example, for a particular section of music, have the lower parts play forte and the first part play piano.
In orchestra, the principal trumpet should not have to play louder to project over the other trumpeters. Section trumpets should generally play under the principal trumpet. Sometimes the balance should be equal. Sometimes the section members have a lead or solo part and need to play accordingly. When playing, the principal trumpet often leads the orchestra.
3. Uniform sound, articulation
In any ensemble, players need to sound the same. They should match the principal trumpet. They may not want to do this, but they must. When they leave the group, they can go back to their own style if they wish. For practice, have one student play something simple, like a scale or arpeggio, and have each player try to play exactly like the model student. You can have the players take turns being the model or use the principal trumpet. When modeling, they can experiment with playing very uniquely and thus making it harder to match (style, articulation, dynamics, intonation, etc.). Have a classmate verbally analyze if they sound like the model or not and how they sound the same or different.
Exercise borrowed from Doug Lindsey, https://www.dougtrumpeter.com/.
Although #1 and #2 address those who play too loud, some young trumpet players have the opposite problem-they do not project their sound well or play too soft. Trumpet players can adjust their bell angle slightly depending on the desired volume of their part. In general, make sure they are not playing into the ground or into a stand. Their bells should be visible to the audience and directed towards them when playing an important part. Orchestral trumpeters in particular will need to project well. Ask them to envision themselves in a large concert hall, and the person in the cheapest seat needs to hear them just as well as those in the orchestra section (concept from Louis Ranger). Please refer to “Projection” in the “Developing” section for more information.